Doubt it. Mergers would have protections for both groups. Likely fences, that would prevent say a CPZ pilot from bidding into a RAH base and vice versa. Either by a certain amount of time, or a set meter the restricts lateral base swaps. Or possibly, they would do a straight SLI, and just let the chips fall were they may. Even then, there would need to be a mass exodus of CPZ pilots to open up spots for RAH guys in our bases. So an LAX CA would not be bumped down unless, an LAX CA left and a more senior pilot from either group then got awarded that base, so absolute worst case scenario, RSV could occur for junior guys. But honestly, I'd bet a merger might spread the seniority around between all the bases, lots of commuters at both companies might want a newly available base. Downgrades could not result from that alone. Only if the the staffing model requires it, and the company announces official dowgrades. Then it would be low seniority first.
So unless a bunch of guys from RAH would want LAX, SEA, or PHX, I doubt it would cause too much issues for any compass folks. Even then, it would take time for all those moves to occur.
Both companies have a decent amount of commuters, even coast to coast guys. So I'd guess things would more or less even out initially.