Time Machine points to wrong data

I recently bought a new Mac Mini to replace my aging PowerMac G5. My plan was to utilize the PowerMac as a file server to the Mini. There were numerous problems during the installation and migration process and, in the confusion, I moved my Powermac external backup disk (firewire attached) from the PowerMac to the new mini. My PowerMac recognized the backup disk over ethernet and started backing up to it. It created a sparse bundle and did another full backup, while I wasn't paying attention. I then moved the backup disk over to the PowerMac again. However, now it will only recognize the sparse bundle backup and not the previous two years of good data in the old backups.backupdb. How do I get Time Machine to revert to the old backups?