GTM upgrade to new version

I'd strongly recommend to go over this thread once,

7 Steps Checklist before upgrading your F5 BIG-IP

And I'd add the below to your existing steps,

1. Re-license and have a copy taken. There's an article on why you need to re-license before upgrade too.
2. Break the sync between GTMs.
3. Disable the interfaces on DC B (GTM-B)
4. Disable the listener of DC B and start the switch the boot drive. (Provided the new image is properly installed on the new volume)
5. Reboot the device. (Make sure you have changed the switchboot location already. Also reboot on new volume command is there [reboot volume HD1.1] )
6. Once the device is up, make sure the configs do match with the GTM-A.
7. Make sure the services on the boxes run, monitor the logs.
8. Enable the interfaces on DC B (GTM-B)
9. Enable the listener of DC B (GTM-B)
10. Turn on the synchronization of DC B, the synchronization name will be a new one here. [Note: synchronization on different versions do not happen]
11. Repeat steps 1-9 again for DC A (GTM-A) and finally turn on synchronization on DC A.