Hi James,
There should be a document about this somewhere.
Found: http://seer.entsupport.symantec.com/docs/273123.htm
As soon as the mailbox is completely archived, you remove the mailbox.
I believe that from that point on, the license is 'freed' as you no longer archive that mailbox.
The archive remains, and keeps the retention as defined in the policy provisioning group.
If you assigned a new policy, the items being archived get this policy, the existing ones keep the retention they originally had.
Gertjan
Regards. Gertjan