Loek,
When your systems loose contact with the PDC after the upgrade, have them join a workgroup, reboot, join the domain again, reboot, as local admin fix up the local groups again, then login in with a domain account.
I am not sure if the profiles from the old domain will be applied or if a new one will be created, but it will get your systems back on the 6.0 domain.
I only tested this with 3 systems...sme, 2k, and XP. There could be other issues or problems here so I would do this in a test environment to make sure it works for you.
ryan