From a Paul Brand newspaper article:
"By the way, Frost also suggests a trick that may remove the old spark plug without damage. Apply a good penetrant like Deep Creep and run the engine up to full temperature before trying to unscrew the plug. The aluminum cylinder head will expand more than the steel body of the spark plug. If you're lucky, the plug may come out without significant damage to the threads in the cylinder head."

But, if the head is really hot, the aluminum will be softer and easier to destroy. So, pour a bunch of penetrant in the plug hole, run the engine, let it cool, check the level of the penetrant and let it sit overnight...
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Keeping it running is better than buying new