Yes, you can just do a straight upgrade to 9.1.
The data and the OS are separate. In fact, I did just what you asked only 3 weeks ago.
The only time you can't do that is if you started from 9.1, you won't be able to go back to 8.3 because 9.1 uses a new version of zfs that's incompatible with 8.3. FreeNAS 9.1 has no trouble reading older zfs formats though.