Windows 2000 Mixed to Win2003. What to Expect?

I have an AD forest with 1 root domain and 3 child domains that originated
from the upgrade of 3 separate NT4 domains. All domains are running in
Windows 2000 Mixed mode as a result of the NT4 upgrade, but all domain
controllers in all domains are now running Win2003 servers, all Exchange
servers have been upgraded to Exchange 2003. There are no Win2000 or NT4
domain controllers left on the network and no expectations to need this
functionality either. I think I'm ready to raise domain functional level to
Win2003, but I'm a little ensure what to expect will happen when I throw the
switch.

The one thing I've been wondering about is how user accounts are specified.
There is both a pre Windows 2000 account name and a regular (new) account
name. My users are used to using the domain\username format for some
authentication. Does this type of "pre Win2000" functionality end when the
domain functionality is raised. Do all users need to be instructed to begin
using username@domain format for specifying user names now instead?


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