If the script does need to be rewritten, that’s barely any time at all before the fall production date.
James Gunn has been updating fans on the writing progress of Guardians of the Galaxy Vol. 3, and according to the latest information, Gunn had completed the script before he was fired.
However, it’s not entirely clear if Marvel is going to use that scrip,t or if they are allowed.
It’s doubtful that Gunn owns the script he wrote and Marvel can probably do whatever they want with it, but there might be Writers Guild rules that throw a wrench into the works.