The new Oshawa station is being built in front of the existing small, grotty station building. I *think* that there will be enough space between it and the existing newish overpass that the existing GO tracks can be extended eastwards through the site of the old station (which would result in another island platform, so they'd have to build another staircase and elevator).
The Oshawa station originally had a simple flat roof; a while back, they built a sloped roof overtop of it. I assumed the flat roof had leakage problems, and this was an easy fix.
One big problem with that station is its location. It is inconveniently far from everywhere. Hence the vast parking lots. (Yes, plural. GO just opened a second lot to the west, on the other side of the Mackie Group building.)
The local (DRT) transit options there are very indirect. If you are going to downtown Oshawa, it is much easier to hop on the eastbound Highway 2 GO bus, which passes directly through downtown Oshawa.
I believe the original CN station was located where the tracks cross over Simcoe St. There was also a CP station a little further north, where that line goes under Simcoe St.
Simcoe St is the spine of Oshawa. Major bus routes follow it, going as far as Durham College/UOIT, and it is still the main road out of town to the north.
Maybe GO should try a Markham- style arrangement, with a pedestrian/PPUDO-oriented station where the line crosses Simcoe St, and keep the existing station with its parking lots, as a car-oriented station, just as the Markham station is more pedestrian-oriented, while just up the line is car-oriented Mount Joy. Same with Newmarket and East Gwillimbury.
Didn't the plan for the Bowmanville extension have a station on the CP line just east of Simcoe St, in the old Knob Hill Farms lands?