I don't think that will happen - at all.
1. They aren't going to rush a goaltending prospect.
2. They like what they have in Dobes and they aren't going to play him the back-up duty.
The most likely scenario is that Fowler will go back to college for year 2, Dobes will continue to start for Laval in the AHL, we will have Primeau/Monty in the NHL, sign a veteran 3rd goalie to a NHL deal like a Hutchinson so Dobes can play uninterrupted down in the AHL.
The following year Fowler will come, split with Dobes as he starts to get NHL starts for call-ups, etc. and the Habs can move out Primeau or Monty at that point to bring Dobes up, allow Fowler to transition to starter in Laval and Volokhin will be ready to come in behind him.