It could have a rematch (even if there is no written clause in the initial contract), if the fight is going to be razor sharp close or maybe even in a draw.
But if it's a convincing win, then maybe not, but if there is a huge money involved then again, they could go and fight one more before going up in weight. So I guess the fans will also have a final say and maybe the promoters are going to take that lead from us if they see a demand for it.
That's true however both sides should secure a rematch clause to make sure that a rematch will happen soon if one will pursue it and so that no one will avoid. There will be demand for sure if the fight is close enough but a rematch won't be possible if one of them starts running after named as the undisputed boxer in welterweight division.