I prefer doing them cold but to each his own. The way I look at is that the steel shank of the spark plug has contracted as much as it's going to.
When loosening plugs never instantly apply a lot of grunt when breaking them loose.
Try it gingerly at first and if it appears to be a bit stubborn try to bump it back the other way; tightening it a bit. Seesaw it back and forth a little.
As long as it persists in not turning freely I would advise that you keep seesawing it until it does. Normally this will bring them out without destroying the threads.
When reinstalling plugs I never use a socket. I always use an 8" long piece of stiff vacuum hose that has an inside diameter just big enough to fit snugly on the tip of spark plug. This prevents any cross-threading. Use some anti-seize on the threads also. Hope that helps.