>The superficial answer is, no problem. You're supposed to be able to transparently upgrade from any SP to a higher one without difficulty.
>
>If you mean, does 6a break anything that worked in 3/4, I don't know.
>
>FWIW you can usually find KB articles describing fixes incorporated into each SP, e.g. http://support.microsoft.com/support/kb/articles/Q241/2/11.ASP
>
>The appropriate keyword seems to be kbWinNTW400SP6a.

Thanks for replies, everyone. It looks optimistic, but I guess the only surefire way to tell is to actually try it. I'll try and find a non-upgraded machine that won't cause anyone to strangle me if there's a problem.