I'm currently running Exchange 2003 SP2 Cluster on a Server 2003 AD Forest (in native 2003 mode), and we beginning to plan the upgrade to Server 2008 AD and Exchange 2010. We have two main sites, one middle-sized office, and a couple of smaller sites which have DCs (which may be RODCs after the upgrade). Currently all of our Exchange cluster is in my main site, but we are considering using the new datastore paradigm for load-balance/failover at the other large site, but this is not set in stone.
Right now we are in the information-gathering and planning phases. I am looking for input of any gotchas experienced while performing either upgrade, but especially the Exchange upgrade.
What were some Gotchas?
What surprised you?
What wasn't documented?
What said one thing but was misleading? (Confusing either in content or severity.)
What is great or horrible about the new system?
What worked well?
What worked poorly?
If you were to do it over again…?
(I know that this isn't so much a question that can be definitively answered, but I'm happy to reward insight and useful resources (not the Microsoft documentation, but Blogposts are welcome) with upvotes.)
UPDATE
A couple items of note:
-We are not currently using OWA (currently only the admins), but it may become more of a consideration with iOS devices.
-We do have a small number of Blackberries in the environment (< 10%).
-In addition to the standard Exchange connectors, we have a third-party connector for Captaris RightFax integration.
Best Answer
The Technet Documentation is really complete. Follow it to a tee. It goes over practically everything regarding getting your 2010 setup going. There's only 4 caveats that I feel are worth mentioning.
First, read up on routing group connectors. The install takes care of creating the first connector, but if you have multiple 2010 servers, you'll need to create additional connectors. Also, don't forget to create receive connectors for your old 2003 setup. You want mail to flow properly while you're in a migration. Surprisingly, I don't remember the docs talking much (or at all) about this.
Second, your legacy OWA might break. This is a situation I ran into during my first migration. You can see a discussion about it on Technet here. Basically, when a 2003 user logs onto a 2010 OWA server, they should be forwarded to a 2003 front end server. This didn't happan all so well. Setting the legacyredirecttype to manual fixed the problem, although making it a little more difficult for the end users to figure out what's happening.
Third, removing your 2003 setup isn't really touched on at all in the 2010 Technet docs. You should follow the information for 2007 available here. When instructed to uninstall Exchange 2003, simply uninstall 1 cluster node at a time. When you get to the last node, let the installer know it's the last node and it will remove the server from Active Directory.
Fourth, I don't remember the docs talking much about ActiveSync. Luckily, the Exchange team has a great blog. They definitely talked about it here.
My first migration went really well. Moved mailboxes over the weekend, came in on Monday and nobody with Outlook noticed anything other than better performance. OWA users were a little surprised, but they learned it quickly.