flash backups are not designed to:
* work between different code revs
* work between different controller hardware models
That's why the master/master-backup solution is far more effective as it replicated the config using the master-redundancy elements, and doesn't mess with the physical interfaces. And that is the officially supported hardware migration process.
While it seems that it should work, it's not designed that way, and there's no guarantee it will ever work nor is there any support for it.
Chances are the most critical elements are there, just fix the network interfaces, then do a diff between the 3600 and 7200 configs once you're done to see if any other major elements are missing.