After you disconnect an Archive Manager server from the Archive Manager Report Pack, you can no longer view historical data for that server in the reports.
This is normal behaviour.
To be able to see the old server in the reports you will need to mark the server as active in the database by running the following SQL statement against your MessageStats database:
UPDATE MessageStats.dbo.T_AMRP_ArchiveManagerServers SET CurrentlyActive = 1 WHERE AMServerName = 'OldServer'
Where OldServer is the name of the old Archive Manager server