I'd certainly take a full backup image of your Domain controller before taking the migration to the next step. You never know what type of problems you'll encounter if critical services such as DNS are affected.
I'd also consider adding multiple IP addresses to your server's network card and slowly start to migrate your workstations to the new IP nework range, to help you identify and troubleshoot the problems that might arise one-by-one, rather than come in the next moring and start running around trying to figure out what went wrong!
I'm going to add multiple IP for each pc and
take image for the domain controller (thanks Chris),
i hope every thing goes fine..
thanks for all your replies, i'll let you know if it work or not.
