The currently committed patch for CASSANDRA-8099 has left backward compatibility on the wire as a TODO. This ticket is to track the actual doing (of which I know Tom Hobbs has already done a good chunk).
Compact value columns aren't being migrated properly in 3.0
Internal auth upgrade dtest failing on trunk
java.lang.UnsupportedOperationException after upgrade to 3.0
Fix upgrade tests from #9704 that are still failing