How do I move users between Instant Messaging (IM) servers?
You can use the Microsoft Management Console (MMC) Active Directory Users and Computers snap-in to move users from one IM home server to another. This process doesn't actually move anything; instead, it changes the value of the Active Directory (AD) attribute that controls which IM server an account is associated with. However, you can't move users to another IM router. After you enable IM on an account, the system stores the router that account uses in the AD's msExchIMMetaPhysical attribute. The only way to change this attribute is to disable IM on the user's machine and reenable it. You can use a script that changes the value of msExchIMMetaPhysical directly to achieve the same goal; this approach would be better if you need to move more than a handful of users.
—Paul Robichaux