The manuals will tell you to always change the plugs on a cold engine. Once you've determined the plug is stuck, you have to slow down and work carefully to make sure you don't strip the threads on the head if at all possible. When I picked up my '89 760 turbo, the PO had boogered the threads on the #3 cylinder. I changed the rest and patiently soaked the #3 plug with KROIL several times over a period of a week or more. Then when I went back after it, I used the following trick to safely retrieve the plug without damaging the head:

Soak liberally with KROIL, PB Balster, or your favorite serious penetrant (don't even think about Liquid Wrench). Start engine, run about 3-5 minutes. CAREFULLY check the temp by putting your hand on the exhaust manifold to feel the heat. You want to stop the engine just as the head is warming up. Hopefully, if you get it just right, the head will have warmed up but the plug will not be at normal operating temp and the hole will be a little bigger than the plug. At least that's what I theorized and it worked for me. Maybe I'm all wet, but it made for a happy ending without damaging the head after discovering a seriously stuck plug (wouldn't budge at all at first).

HTH,

someguyfromMaryland