If the diagrams are still being looked at for approval, surely the timetables are subject to change between now and December as there may be some services that cannot be run. It does appear there are currently a number of 'illegal' moves that will see these diagrams recalculated, potentially multiple times, until correct.
On the GN side, many drivers will be moving depots. There is now some training going on to fill gaps with 387 signing. There seem to be plenty of drivers trained for 700s but without the knowledge, and it seems 717 training will be pushed back as a result.
I think that with the timetable so important, the introduction of the 717s will have to fit around everything else.