I have two exchange environments and was lucky enough to get a different exchange guy at each site to do it and they both did it differently. The first one was done by making the 2010 the cas and primary server and creating the legacy.domain.com, basically migrating owa and active sync to the 2010 server and relegating the 2003 server to secondary servers. The other place has been done differently, the 2003 server remains the primary server and the 2010 server has no owa or active sync working during this transitional stage as the guy said he could create a new subdomain for 2010 but its better to just decommission the 2003 server as less work and better method.
Both sites i can move mailboxes across no problems. But we have software that relies on exchange, contact db and dms filing systems. These need to be made to work with the 2010 and service accounts moved across. There is no guarantee that 2003 users will continue too work after making changes to services. Then we have the blackberry and active sync mobiles that are impacted by moving mailboxes.
We had to stop moving mailboxes at one site because the users were still on xp and 2003 and this was leading to lots of problems after mailbox moves, we just rolling out 2010 and win 7 and the plan was to move mailboxes as they move across.
The other site i have to make all the changes on a friday night to the services and bes and then hope that 2003 accounts continue too work ok, otherwise we have to rush too move all the users across. We have to move the users anyway so im just going to try move everyone across.
The reason i asked is because I have to deal with this annoying project manager recently that was saying how the exchange project was a failure because we never met his project deadline. I tried to rationalize with the man but he said that he has seen other sites where exchange 2010 went through without any problems.
