No, you can't. You must reinstall W2k from scratch.
If you're doing this for cosmetic reasons, I believe XP can be arranged
to look almost identical to W2k, and it might be worth trying to do
that. The XP cutesy-poo default presentation scheme seems to really
upset some people.
If you've got a technical reason to go with W2k, like hardware or
software compatibility, the above is irrelevant.