Our current email address policy in exchange 2010 has way too many email address formats so I'd like to trim it down.
If I edit the policy and remove one of the formats, will that remove the corresponding addresses from my existing users, or will their extra addresses remain?
Ideally, I want them all to remain.
It has not removed the addresses for users when I have done this in the past. Obviously, it will not create them for new accounts added after the policy is removed.
As in all things, you should test this for yourself to be sure
http://technet.microsoft.com/en-us/library/bb232171.aspx
Look at the "Behavior of Recipient Policies" section, it explains what happens when you modify these policies in Exchange 2010.