it's only happened once before, in 2020, and the conditions were very different: Sven, the then-dom, was badly wounded by a fox, so Chirk, barely 1 yr old, just had to take over the role, and when he recovered, Sven just accepted his new role as sub as Chirk wasn't prepared to give it up again.
In terms of how dominance is established and maintained here, up till now it's boiled down to the subs running away from the dom as and when the dom can be bothered to chase them (which is very infrequently, especially as the sub birds don't challenge the dom). He does it now and then just to remind everyone who's boss I think. There's no fences so no potential traps and they all know a myriad routes through the undergrowth.
There's no possessiveness over any of the hens - the hens decide who they want to mate with, usually (the odd head grab does occur) - and last year's clutch proved Venka was mating with both Chirk and Phoenix - but sometimes one or another roo will interrupt another's mating attempts. In almost all cases the worst case scenario is a chase until they're exhausted. Killay has been testing Chirk for months, provoking him and then running just far and just fast enough to avoid being caught, almost skipping aside, reminding me of football players dodging each other's attempted tackles. Yesterday he stopped running, and they were both head down and flared hackles on and off for a while. Chirk was jumping and lunging with feet first, but Killay was dodging and Chirk usually landed on his back. It didn't take long before he was worn out (he is 4), and then it was Killay chasing Chirk, and it was all over bar the shouting.
At roost time Chirk tried to sneak into one of the medium coops, but Killay saw it of course, and when Killay approached the ramp, it was obvious from Chirk's voice and the shavings being expelled that he wouldn't be kicked out without a fight; Killay just walked away.
I shut the coop door to avoid any further high emotion until Killay had gone to roost, then reopened it, and a couple of the hens that were not enthusiastic about roosting (posted photo of them on high bar last night) chose to roost with Chirk (they could have gone to another coop).
This morning he didn't come out for breakfast till everyone else had moved on, and I guess he'll be spending the days on his own or with the other older members (both Maria and Venka sought him out yesterday afternoon) till the dust settles. And then we'll have to wait and see. Sven remained with the flock for 3 of his 4 years as sub. Amadeo and Fforest are with the flock, no issues, and as long as they don't challenge Killay, I expect it to remain that way for them. Phoenix sired quite a few of the current flock; being sub just means they don't get first dibs at food time and need to move on if the dom comes along, it doesn't mean they live unfulfilled lives.