Ever since I first learned a few months ago that Exchange 2010 will not support Single Copy Clusters (a high-availability model which has served us quite well to date), I’ve been scheming to figure out ways to transition to 2010 in a way that impacts end-users as little as possible, a task made more difficult by the lack of information about upgrade paths which has come out of Microsoft to date. I had even come up with an elaborate transition model built around Standby Continuous Replication (SCR) which would have made the transition almost transparent to end-users, provided that Exchange 2010 supports in-place upgrades from 2007, or that an Exchange 2010 mailbox server can serve as an SCR target for Exchange 2007. Alas, a new webcast from Microsoft has dashed those hopes, as both of those options are explicitly excluded from officially supported upgrade paths.
The Exchange engineers really dropped the ball on this one. Allowing an Exchange 2010 mailbox server to be an SCR target for 2007 would make transitioning quite painless. Alas, it is not meant to be. (Of course, I understand WHY they couldn’t do this. Exchange 2010 marks the first time that the ESE database schema for mailbox storage has been changed substantially since the introduction of Exchange. Thus, the 2007 and 2010 mailbox stores are incompatible, so it would do no good to replicate a 2007 database to a 2010 server unless some sort of database format conversion could be done on the fly.)
The supported transition path involves building new Exchange 2010 servers and performing move-mailbox operations on each mailbox, which, fortunately for us, is an easily scripted operation with PowerShell. There is one bright side to this. Exchange 2010 introduces the concept of “Online Move Mailbox.” In previous versions of Exchange, the move-mailbox operation rendered a mailbox unavailable for the duration of the move operation. With this enhancement, the mailbox remains continuously available except for a brief interruption when access is handed off from the source server to the destination server. This online move mailbox feature is supported for moves from Exchange 2007 SP2 (a forthcoming service pack) to Exchange 2010.