It seems like there has been some conflicting information over the years on the best (least downtime) method to upgrade a redundant pair. It used to be recommended to upgrade the Master first. This was because the project state would be Incompatible if the Backup was upgraded first and clients tried to switch over.
Recently, I’ve seen a support topic from Travis that recommends upgrading the Backup first and transfer clients over to the Backup after its reconnected. Then, upgrade the Master.
I just tried this with an upgrade from 7.7.3 to 7.7.4 and sure enough the clients wouldn’t transfer to the Backup after it was upgraded because it the project state was Incompatible. Am I missing some kind of override settings or something?