Valve Repair

            There are basically (3) types of valves used in pools today:
                1.) Cheap plastic valves with no repairable or replaceable parts. These last about 10 years and then need to be cut out and replaced because they no longer work or their handles crack off. You are looking at plumbing a new valve in place.
        2.) Brass, or other metal, valves, including the old brass gate valves and metal ball valves. These normally last about 15 years, and again, they are disposable as well. There are no serviceable parts. You are looking at replumbing a new valve in place.
        3.) Lastly we have the "serviceable" plastic valves, with screws and gaskets to hold the assemblies together. The best of these is the "Jandy" "Neverlube" series of valves and gate valves. These are the valves I like to use for a number of reasons.
            a.) If the valve has a warrantee issue, you just replace the part of the valve that is bad, and you are back in business. Labor, and consequential parts are never included in a warrantee, although I guarantee my installations forever, for you, the next owner, ... I, personally, offer no warrantee on parts. That's the manufacturers warrantee that covers those. If my installation is what failed, I cover absolutely everything. (Note: in 30 years I have had one installation issue. I covered every aspect of it.)
            b.) Most valves are a part of a manifold, or set of valves attached by plumbing. On many, there is a suction side manifold, and a return manifold. The suction manifold is where the suction to the pool / spa, main drain, skimmer(s) and cleaning system is adjusted. The return manifold is where all of the filtered water is returned to the pool. It adjusts how much water goes to the return eyeballs, floor head system (If installed), pool, spa, aerator(s), ... If you have to replace one, you have to cut all of the pipes to all of them in order to do so. You don't have to worry about this with the high quality plastic valves. Only the guts need to be replaced. Replumbing is not necessary.
        If you have a bad, high quality plastic valve, you, the homeowner, can shut off the equipment at the switch or breaker (So that the pool cannot come on, unscrew the guts on the valve and bring it down to the pool supply store and get a replacement valve assembly (For the neverlubes) or order a replacement for the other brands of higher quality valves. You will need to bring the parts in so that you can match apples to apples, ...
        If you are replacing the cheap valves because you must do so to move, and your pool will not pass the home inspection without them, put in more "cheap valves". Yes, you need to replace them all at the same time, besides, they are usually all broken, or breaking, at this point. No, I wouldn't use unions because it won't be you who pays to have everything cut up and put back together if there is a warrantee failure, or a failure years down the road.
    Otherwise, I would use the high quality valves, and put them in with unions wherever possible so that "should a housing be damaged", you can slide out the old, and slide a new one in it's place. Manifolds are on the upper limit of plumbing repairs as you have to plumb in multiple fittings all at the same time, and messing up n one, will result in a leak, cutting out all the stuff you just installed, buying all new fittings, and trying again. My suggestion is that you call a pool professional.