There's a process in place via the ALPA manuals if you've got a dispute. I don't know the exact sections or details (but the reps will). Odds are if they sue without going through that process first, it's gonna be an expensive thing to see tossed out.....
Even if it's not tossed out, once the lawyers get their chunk of the deal, they'll probably be taking home about the same amount as they would under the new method anyway. Seems kinda silly to me. Then again, my signing bonus would be equally bad either way since I'm in the middle of the road.