The only things effected in changing platforms elevations is the snow melting system, platform edge and the elevator.
As noted, the existing platforms can be change in phases that will require the existing shelters to be removed if attached to the platform or the floor to be raised, depending on the extra height require. You could have a slopping ramp to the shelter as well to the stairs and elevator.
Not my field and leave it to others, but I expect the existing snow melting system will have to be rebuilt to deal with the new elevation for the platform. This is very costly as well time consuming to do.
Trying to remember and stand to be corrected, but most non high platforms were like GO platforms where the floor of the the commuter rail coaches floor were the same height of the platform with an gap plate in Europe. Intercity trains had steps to get to the low level platform that were very common for a large number of countries.
Other than time and money, existing platforms can be raise to match the floor of GO coaches.
If you want to go high floor, major rebuilt for not only for everything's connected to the platform, but also for every coach in the current fleet. The existing doors would have to be relocated to the end section with the removal of most of the seat forcing riders to use stairs to the lower level and what do you do for washrooms for accessibility riders?? Would not recommend this at all.