Already mentioned from Karsten , but I want to confirm it . here the way what we did and what was working :
You can jump straight from 7.2.5 to 8.2.5 with zero downtime.
However I would suggest you to perform the upgrade in a scheduled time window as the nodes will temporarily run different software versions during the upgrade process.
If you follow these steps you should be able to successfully carry out the upgrade with zero downtime:
1.Copy via tftp the image to the primary ASA and set the new image to be used
(for example: boot system disk0: /asa825-k8.bin)
2. Copy via tftp the image to the secondary ASA and set the new image to be used
3.On the active firewall run: failover reload-standby
When the standby unit has finished reloading, and is in the Standby Ready state, force the active unit to fail over to the standby unit by entering the following command on the active unit. Use the show failover command to verify that the standby unit is in the Standby Ready state.
4. Check the status of the secondary unit by “show failover”, when it will be in “Standby Ready” state, run “no failover active” on the primary ASA to force a failover to the secondary firewall, which now will become active with the new software version.
5. Reload the primary firewall.
Reload the former active unit (now the new standby unit) by entering the following command:
6. When the primary is in the “Standby Ready” state, issue the command “no failover active” on the secondary unit to put the primary unit back to the active state.