Operationally, it makes the most sense to use the Siemens trains on all of the Ottawa-Montréal-Québec City trains first, as it will allow VIA to tighten up their schedules and do away with the backing-out-of-Central procedure that most of the trains have to do - and which costs them 10+ minutes per trip.
Once all of those trains have been be converted, the next thing to look at is the trips that feature changes of direction each day, and especially in places like Ottawa where changing the direction of a train can be problematic due to the schedules of other trains arriving and departing on the single track to the east.
Dan