I have a network of around 70 machines, currently with two DCs both running Windows Server 2003 (DC0 & DC1). DC0 is a five year old Poweredge 1850 and has recently become increasingly flakey, and in the past fortnight has fallen over twice.

I want to replace this machine, but I'm cautious as there is huge scope for this sort of thing to go wrong. The way I imagine doing this is building a new machine then doing a DCPROMO and running three domain controllers for a month or so until I'm happy that everything is working as it should be before retiring the old machine.

Particular areas of concern are the replication of roles from the current controllers (GP settings for instance) and the ramifications of switching off the machine that has, up until now, been the 'primary'.

If there are compelling reasons to use Server 2008 I'm willing to do so, however I don't know if this would cause problems with my exisiting 2003 machines.

Any advice on best practice or previous experiences would be most welcome.