You have a single Exchange 2010 SP1 server "Ex2010.domain.local" running HT, MBX and CAS roles. You move a mailbox from the Exchange 2010 SP1 install to a new Exchange 2010 SP1 CAS Array installation "CASArray01.domain.local".
Outlook 2003, 2007 and 2010 will still points at the mailbox "Ex2010.domain.local", it will not automatically update and point at "CASArray01.domain.local". This is because the RPC CA service doesn't respond with a "ecWrongServer" like previous versions of Exchange did.
Microsoft is aware of this issue however their is no easy resolution in terms of a little code tweak.
How do I prevent this from happening:
Microsoft has been telling customers for a long time to always create a CAS Array even if they have one server - they need a one server array. If all servers single servers were setup in an Array from the beginning this problem would not exist.
Is there a way to force all Outlook clients to automatically perform a full re-autodiscover and attach to the new server?
Yes - if you remove the "Host A" record from DNS for the old Exchange 2010 server old Exchange 2010 server "Ex2010.domain.local" the Outlook clients should do a full re-autodiscover and attach to the new CAS array "CASArray01.domain.local" automatically.
This however is generally impractical especially when you want to stage your mailbox move slowly instead of attempt the big bang approach where all mailboxes are moved in one hit. For small server migrations this is a workable solution.
Is there another work around other then removing a Host A record from DNS?
Yes - create a PRF (Outlook Profile) configuration file to automatically update Outlook to point to the new server. You will need to script this out to ensure it automatically runs on all workstations.
Please look at this article for information on pushing these settings via Group Policy or Script: