The usual way to do it was (and probably still is) to set up the new machine
as a BDC, let it synchronize with the old PDC to get the user info, etc., and
then promote it to PDC, which will demote the old PCD to a BDC - then you can
remove the old machine if you wish.
I can't remember at which point in there it is best to rename the new machine
(probably after the old one is gone) - or you could possibly set up a netbios
alias so that it answers to the old name as well as its new name, until you
get all clients, etc. set to use the new name.

|How do I replace a PDC computer with a new computer.