The rules on seniority are set out to stop disputes and personal opinion. However, if the senior official does not feel that they should take over for various reasons - injury, etc; they they waive their entitlement and the other AR takes over. The important factor is get the match finished. However, having arrange another appointment after this one, is not a valid reason to stand aside. The FA would take dim view (especially if the later match was of a lower standing, i.e. a juniors match).Ah yes, blame the early start for me not reading point 1 first!
Still, I don't really like the idea this is a strict thing that must be followed with no deviation. The ex-L2 in question had stepped down due to the fact his mobility had decreased, and was also grumbling about having been given another game the following day. So both he and I were happy to have him on the "junior line" and up until this thread, I would have assumed that means the other AR takes over in an injury. The only other time I've worked with this guy, we were both assistants and the referee made the exact same decision - put me on the benches and him on the other side, again I was ready to come on if needed in that situation.
I've turned up to games while on the road to recovery from an ankle injury, or with a slight stomach bug, or having slept badly the night before - happy to go up and down the line for 90 minutes, but not necessarily feeling mentally or physically up for more than that. On those occasions I've asked to go Junior, and unless there's a brand new deer-in-the-headlights L7 on the other line, it's been the correct decision to go along with that, even if I am technically more senior. Referees aren't robots, certainly not below PL level, these soft factors have to be considered.
What you have to remember @GraemeS is that who does bench side is your choice, we are talking about who takes over and the League rules.