literal answer: 50-50
pragmatic answer: the flight is midafternoon ... imo, if you call ~noon and say you won't be able to make it to PAE in time, your chances are far better that they will offer you a reasonable option -- one of the five evening flights from SEA, or an early evening connection out of PAE (~530 over LAS, ~600 over PDX) -- than if you call at T-24 hours