Install the Support Tools from Microsoft if you have not installed already
- Create a new Administrative Group In Exchange  System Manager if you have not already.
- Start -> Run -> C:\Program Files\Support Tools -> adsiedit.msc
- Expand CN=Configuration,DC=example,DC=com, expand CN=Services, expand CN=Microsoft Exchange
- Expand CN=Organization Name (where Organization Name is the name of your Exchange organization).
- Expand Administrative Groups (if administrative groups are enabled), expand your servers current administrative group (for example, expand CN=First Administrative Group)
- Expand CN=Servers
- Right click on your server and select Move
- Using the navigation provided, traverse the tree down to your desired Administrative Group
 
 
Remark:
This appeared to work for me in a lab environment. I even tried moving mailboxes between the servers and it worked with no snags. I don’t suggest doing this in a production environment simply because MSFT doesn’t officially recognize this method and there may be some kind of replication issues that I haven’t seen come up yet.