Within the 8.4 release train there are no issues upgrading that I'm aware of.
To upgrade to 9.1(3) or later you would need to take an intermediate step as described in the release notes here.
In either case you should also upgrade your ASDM to the most recent 7.1(4) release.
By the way, it's not called IOS - it's just ASA (Adaptive Security Appliance) software.