The changes are as follows:
Lakeshore East Line: Tracks 25 and 26
Lakeshore West Line: Tracks 5 and 6 or 7 and 8
With the Lakeshore trains, due to the number of departures on these two lines, some train will not depart on the tracks mentioned above.
Barrie Line: Tracks 11 and 12
Kitchener Line: Tracks 9 and 10
Milton Line: Track 4
Richmond Hill Line: Track 13
Stouffville Line: Track 27
For the Barrie, Kitchener, Milton, Richmond Hill, and Stouffville lines, the above tracks are scheduled to apply to all departures.
GO Transit has provided a list of the new platforms, and especially for the Lakeshore lines, please double check your departure here: GO Transit Platform Changes.
However, the best option is always to check the departure boards at Union Station prior to heading up to a platform for your train. It is always possible, due to availability of consists, the continuing work at Union Station, or other issues, that your train may depart from another platform.