@Aeoran AFAIK, the XAPI database gets wiped whenever you add or remove the host from a pool. You may be able to restore metadata to the old master once it is no longer belongs to the pool, but I can't guarantee that this will work or not produce other issues.
If you don't have backups of the VMs, then you should be able to copy the VHD files to another location by accessing the directory /run/sr-mount/<SR UUID>/ on the old master.