As someone in the middle of a transition between Exchange 2007 SP3 and Exchange 2010 SP1, I have to ask; MS Exchange Team, what were you smoking when you came up with the interoperability options here?
2003 to 2007 was pretty decent, you could deploy your 2007 CAS servers more-or-less directly in place of your 2003 OWA front-end servers and they would happily serve 2003 mailboxes up to people. Management tool interop was lacking a bit, but that was excusable as 2007 was, at least, a totally new set of tools based on .NET/Powershell and they generally wouldn’t let you do anything to a 2003 mailbox that would break it horribly.
With 2007 to 2010 though, you can’t do any of that. You have to keep your 2007 CAS and HT servers around because the 2010 ones won’t play nice with 2007 mailbox servers, but then the 2010 CAS servers can’t connect to 2007 mailboxes. Except they can, if you copy the 2007 binaries onto them. Except that only works if there isn’t a 2007 CAS server in the same AD site as the 2010 CAS server. So, short of creating new AD sites for all your 2010 mailbox servers you have to have at least one AD site with both 2010 & 2007 CAS servers, which works OK internally, but if you’re publishing to the internets (and who isn’t these days?) then you start to run into issues. There are various articles about this (like this one) but basically you have to create a new internet-facing DNS entry for the 2007 CAS, move the 2010 to the old internet-facing DNS name, change the ExternalURL value on the 2007 CAS to match the new DNS name, change the internal DNS so that there’s an entry for the new DNS name (because Exchange treats other AD sites as “External” as well), buy a new trusted SSL Certificate for the new DNS name (or wildcard it) and pray to God that you’re only publishing 2007 CAS servers from a single AD site; otherwise it gets so complicated that your brain will try and kill itself to escape.
If you think that sounds bad, then wait; it gets better! You can’t use the 2010 management tools (which are now 64-bit only – unless you enjoy Powershell Remoting) to manage 2007 mailboxes. Well that’s not entirely true, you can so some things, just not everything and it’s the same with using the 2007 tools on 2010 mailboxes. It’s all documented on The MSExchangeTeam Blog and it’s a huge mess – if your helpdesk staff (or whoever else manages your mailboxes day-to-day) aren’t really on the ball there’s a good chance you’ll run into problems while the 2007->2010 transition is in progress.
All this is in addition to all the subtle syntax changes they’ve made to a lot of the management Cmdlets so that things which were valid commands in 2007 don’t quite work properly in 2010; I’m looking at you, Import-ExchangeCertificate – let’s go from importing a CA-supplied .cer file to importing the binary contents of said file as a Byte Array supplied as a command line argument, because that’s much easier.
I really like Exchange – 2007 SP1 onwards was top-notch – and 2010 has some great new functionality, but they’ve really dropped the ball when it comes to installation, upgrading, migrating and transitioning.