Actually, what I've done is install windows 7 on my old XP machine, and since I went x64 and used to have x32, there was no upgrade path.
I used the User State Migration Tool (USMT) to move a lot of stuff, and for the stuff it moved, it fixed the file/directory ACLs, but not everything. Quite a bit of data was just left sitting on the drive.
All that leftover data has ACLs that were relevant to the XP system. In particular, most files are owned by the primary user account of the old system.
So how can I replace one SID in the ACLs with a new one, remapping all the permissions from the old user to the new user?