Yes, it is possible to "restore" a filled in pool. And, for those interested, a good liner company can produce a liner to fit practically any pool design and shape. I do however, have a few comments/suggestions/questions you may whis to consider in planning for such a job ...
The first question you should ask (if you can get an answer to it) is: "Why was the pool closed in the first place?" Was it simply an economic thing ... where it was hardly ever used and they wished to be free of the expense? Were there actual physical issues (leaks and resurfacing needs) that they did not desire to address. Im many areas ... the cost of "filling in" vs the cost of "resurfacing" are close to the same and filling in is often cheaper. Plus, after a "proper" fill in ... there is no further expense, where with a resurfacing, you have the annual maintenance costs continuing.
At the equipment end of the lines - were they simply cut off at or below ground level (most normal) and left in place? Or were they removed to the deck edge? Was the skimmer left in place? filled in with dirt or concrete?
If there was a light ... are the existing electrical lines still in place or were they removed?
Personally, I would do the following "First": Locate the lines underground where the equipment used to sit.
Clean out the skimmer. use an air compressor or high pressure air/water source to "flush/blow out" the line and pressure test it for soundness. A good pressure test here is a fair indicator that the underground lines "should" all be in good condition.
This can be "fun": Dig down along the inner walls of the pool and locate the return fittings. Most Gunite Pools will have two standard pool inlets (pool water return lines) - 1 in the shallow end, near that end of the pool and one in the deep end, also near that end of the pool (Some pools do have more though - smaller pools may have only one). When you locate one of them, you can let it "help you" find the others and identify which line at the equipment is the one which feeds it. Feed air or water (under pressure) into the open line and see which line at the equipment it comes out of. Plug that and attempt a pressure test ... if the return fittings are individually plumbed it should hold pressure. If the returns are all linked and fed off a single line (more common) it will not, but the other fittings in the system may "sound off" for you - identifying their locations. As you find one, plug it and reattempt the pressure test. When you find the last fitting, the line will hold pressure - unless there is a leak. You should be able to guess from the reaction whether you have found all of the fittings or not.
Most often, there will also be an additional dedicated return line in the side wall of the pool, right in the middle of the wall. This line will have been installed with a pool cleaner in mind. These cleaners mostly used a booster pump at higher pressure so needed their own line. Depending on the type of cleaner (if any) you plan for the pool you may or may not "need" this line. It can be either repaired or left plugged off later. A pool professional at a local store can explain all of the options for this line - they are too numerous to go into here.
Pool return lines are (normally) located 12 to 15 inches below the pool coping (though they "can" be anywhere in the pool - even the floor).
Once you are satisfied with the underground lines and plan to go ahead with the job, anticipate this:
1 - Once the dirt/soild has been removed - pressure washing the walls of the pool to remove the vast majority of the remaining soil and crud. You will likely want to place a "dirty water" or "trash" sump pump at the bottom of the deep end of the pool during this operation ... pumping out the excess "muddy" water as you add it. You will find yourself using a shop vac to remove what the pump does not get.
2 - The pressure washing will also identify the drainage "bore/weep" holes in the surafce of the pool as well. These will need to be cleaned of soil and other material so they can be filled with cement and replugged.
3 - From the "equipment end" back flush all of the pool lines - not only will this identify which is which for you, but is will help clean them of any soil impacting. Once identified - Mark them! Once clean - re-pressure test all of the lines ... Most pool lines are rated to 70 - 80 pounds of pressure. Each should be able to hold 40 - 45 lbs of pressure if properly sealed (without losing pressure) for ten or fifteen minutes.
4 - Inspect the main drain fitting for any damage - there should be two openings - one in the side wall of it to which the pool plumbing is attached. One in it's floor which drains directly into the ground - often a drainage pipe is attached beneath this fitting. if this is choked with dirt and debris it will need to be cleaned at least to below the plastic ... this opening is for the attachment of a "relief valve". What a relief valve does is to actually allow ground water "into" the pool during high water table conditions. This is a protective feature. Essentially a pool is a big "boat hull" or "bath tub" sunken into the ground. Should the ground water pressure around the pool be greater than the weight of the pool iteslf it can literally "float" itself out of the ground ... in a liner pool pockets of water will raise up under the liner lifting it from the floor - in a solid pool - the whole thing can float up.
5 - Inspect the pool light niche (if any) for deterioration/damage/tightness around the edges. Should there by problems you have a couple of options - repair the problem and reinstall an original style light .... extend the wiring conduit and "fill in" the rest of the pot around it (newer lighting is available which essentailly "surface mounts" on the pool face and needs only a conduit to feed the wiring through) .... fill in and cover over the light pot and "do without a pool light". (There are lighting options available which can hang over the pool edge and down into the pool these days)
Inspect all inlet fitting and the area around the skimmer opening for damage/water seal. You won't always "see it" if there is a leak, but normally you can. Should you see any type of cracking or "gap" along the edges of the skimmer face (where the plastic joins the gunite/plaster) you can use a simple test. try to insert the tip of a pen knife (small pocket knife) into the suspected crack/gap. If it goes in more than about 1/8 inch, figure you have a leak. In this case, the leak can be repaired by chipping out a fair amount (about 3/4 to 1 inch from the plastic itself and sloping down towards the plastic to a depth of at least 1/2 inch all the way across the bottoms and up the two sides. You back-fill this carefully with a marsite plaster mix that is wet enough to work, but not wet enough to "sag" or "run" when you press it into the side walls. Press it in firmly and tightly all around, smoothing it carefully to the surrounding plaster and the plastic of the skimmer.
This next part is for "Filledpoolguys" as well:
"PLAN ON HAVING TO RESURFACE THE POOL" ... No matter how careful you are, the surface coating on the pool will receive or will have received some damage - not to mention staining from having been buried and chemicals in the soil. As for the kind of coating you apply - that is up to you ... the options are numerous. I will say that any recoating method will need be preceeded by a thorough degreasing and an acid cleaning. Otherwise the new coating may not "bond" properly to the old surface. As for recommendations ... I "NEVER" recommend painting a gunite pool. No matter how thouroughly cleaned and prepared, I have never seen paint last more than three or four years without peeling or causing popping of the plaster beneath it (it usually begins happening only two years after the painting of the surface.
If there is evidence of the pool having been painted in the past ... you "may" have to consider sand blasting for any other resurfacing method to bond properly. If you replaster the pool I know you will. With a Pebble-Tech finish I do not know ... I do not know how "picky" the resin base is in a pebble Tech pool.
For information - if "replastering" - the only plaster I know of which will withstand pool chemicals over the "the long haul" without degrading is "Marcite Plaster" (plaster made/incorporated with marble dust). Other plasters I have seen tried tend to erode/chemical away more rapidly. A well done, replastered pool will have a finish that will normally last 20 to 30 years.
Anticipated retiling around the upper edge of the pool. You may get lucky as original tiling jobs can be extremely long lasting and sound. Replacement tiling, in many cases, does not seem to hold as well - maybe it is the difference in craftsmanship between thirty/forty years agon and today? If the tile appears sound and no looseness develops with the pool cleaning I have described - I would not mess with it. Sound like good stuff to me. If there is looseness or damaged tile, plan on a tiling job in conjunction with the refinishing - it will last longer than one done "after" the resurfacing.
I do not normally recommend "completely tiling" a swimming pool - but should you wish to try, it should be a non-glossy tile. Glossy/smooth surface tiles can be too slippery for a swimmers footing.
Now - PUTTING IN A LINER: As I said, a "good" liner company "can" mae a liner to fit inside of a gunite pool - but every new angle and odd cut adds substantially to a liner's cost. PLUS: the pool inlet, drain fittings and light mounting niche will need be changed or adapted to a type that accepts a pool liner - a fitting with a faceplate. Around the facing of the skimmer opening you need to install the same type of moulding/tracking used when installing liners around step coves or seats - for a proper water seal. If there are inlet (plastic or tile) steps mounted in/as part of the pool wall - they will need to be chiselled out and then filled to "flush" with the surrounding walls. Trying to measure and cut a liner to "fit" these steps is more problematic than simply buying a standard pool ladder (not to mention how much each of those steps individually adds to the liner cost). Tracking will have to be installed around the upper edge of the pool walls so that a liner can be installed. The tiling around the pool walls should be removed and the area filled and smooted the the surrounding wall surface (to protect the liner and it's installation). Any roughness in the pool surface will need to be smoothed down and any "chips in the surface" filled (if the surface is not "smooth", every imperfection will show through the lining once installed - and any roughness is a potential failure point for the liner itself - through abrassion and possible cutting).
I think I have covered the high points ... though you will find other expenses as well.
Ray10311 is an experienced pool professional with 25 years experience