Forum Discussion
My real world experience - we upgraded from 10.2.4 to 11.2.1 - and it was very rough. The transition from the HA approach for 10.2 to the traffic-group based approach in 11.x didn't go well, and we ended up booting back to 10.2, planning an extended outage window, and then upgrading each as a standalone device, then establishing HA after each device was in standalone mode. We had customer service involved, but were on a closed network, and couldn't have them remote in ... that didn't help. But even with all the diagnostic information we could provide, it was never clear what caused the failure.
It really made me wish we had a 3rd device that we could temporarily co-opt to handle the load, even though that would have been significant work.
That's just one datapoint, of course ... and I haven't done a 10.x to 11.4.x+ (there was a major architectural change in HA at 11.4).