Ok just a short post on this topic.
When upgrading from Exchange 2007 to Exchange 2010 you can let your E2010CAS redirect your users that are still hosted on the 2007 server to a legacy record. This will enable you to migrate over time and take it easy and just have a great and good time with your upgrade.
Well to make it short, IF you are using mobileiron or any other mobile device management system that takes over the active sync part. DO NOT, I repeat DON’T run Set-ActiveSyncVirtualDirectory –ExternalURL legacy.domain.com or change the external EAS record in the ECP.
This is because the phones are so smart now days that they actually updates the server record to legacy.domain.com and tries to logon to that server. And guess what, well since they are managed by another system they will not be able to logon and you will loose them.
So if you did set your external url to something just run:
Get-ActiveSyncVirtualDirectory -Server Exchange2007CAS | Set-ActiveSyncVirtualDirectory -ExternalURL:$null
This will reset the automagically forward thing and just proxy the EAS instead. see this post for more info http://www.stevieg.org/2010/01/solving-iphone-and-exchange-20102007-coexistance-issues/
And then reset their EAS server record via the MobileIron profile. And its all good again.
And one more thing, read through http://blogs.technet.com/b/exchange/archive/2009/12/08/3408985.aspx for some great info on EAS upgrades.