first off it's not called "degrade" that implies wear and tear on the current CPU. Switching to a lower-power/lower-performance cpu is called "downgrading"
Sry, that word just stuck out like a sore thumb for me...
answer is no, someone correct me if I'm wrong, but the OS, once activated, locks itself to the Motherboard's ID, so you could swap in new cpus, video cards, and even add another HD provided that the motherboard is still the same one.
I recently upgraded my Dad's comp from a A64 3200+ venice to a A64 X2 3800+ Toledo. It was as simple as removing the old one and tossing the new one in (plus a new cooler too, but that's irrelevant of course)