There are two basic questions:
1. Is the new client handled in Avamar as original, has same client id??
2. How long is retention policy??
If client has same id than you should be able to view backups from original, if not, you have to check correct one from Management console and restore data from old client to new one.
If all backups has become expired, than it is question of setting Avamar, it should not recycle last backup of any client by default so you should so one at least. If all backups became recycled, than there is no way to restore it, because Garbage collection deletes chunks with no link to valid backup.