While some folks advocate the 8.3 step, you can go straight to 8.4 from 8.2.
Staying in a version mismatch state is not recommended since any commands in the primary will be replicated to the secondary but, if not in 8.3/8.4 syntax (for the items whose syntax changes in the upgrade) you will be left with configuration bits that aren't compatible on your standby unit in the event of a failover. (During the Secondary - Standby unit reload, the 8.2 configuration is parsed and running-configuration syntax changed as necessary) As long as you understand and accept that, you can run for an extended period with the mismatch - you just expose yourself to risk in the event that you have made configuration changes and an unplanned failover occurs. That's why only a 'short period' is the recommended period for such a state.
In all the upgrades I've done, I always upgrade the Primary unit (after validating the Secondary - Standby unit's upgrade went OK and making it Secondary - Active) during the same maintenance window. When it goes well it's a 10-15 minute process for the both of them.
The TAC engineers are well-versed in this process and are very able to support you during the process if you open a case proactively.
