I think someone would actually have to be fired (or their contract allowed to expire without renewal) in this purely hypothetical situation.
NCAA sets limit at 9 assistant coaches for FBS programs, I think (plus 2 graduate assistants, plus strength & conditioning, nutrition, etc. staff). We have 9 assistant coaches now. Peyton can't be a GA and wouldn't get hired for S&C/nutrition. Therefore, one of our current 9 asst coaches would have to go, under this scenario.
Since Peyton would undoubtedly be working with the offense, OC is one real possibility. He might work specifically with QBs (and maybe WRs too), so the Pass Game Coordinator / WR Position Coach is a second possibility. Running backs aren't his thing, so Gillespie is probably safe (plus, his position group is probably the single best-performing of them all right now). Elder is safe at Tight Ends & Special Teams. Final possibility is DeBord shifting to OLine & Assistant Head Coach, which would mean Mahoney out.
So that's it. The 3 possibilities are DeBord out, Azzanni out, or DeBord shift and Mahoney out.
Having said all that...it's just a hypothetical. And not a very realistic one, primarily for the reason VolAllen already mentioned.
p.s. I don't know of a single thing in the NCAA rule book stopping a person from volunteering their time to help out a football team at practices, during spring and fall camp, etc. Look for something like this as far more likely than any firing or hiring scenario.