Maybe. Speaking of the University of Toronto:
- In a professional program, such as Engineering, no. After first year, you can change disciplines (e.g. civil to mechanical), but it depends on how you enrolled. They’ve changed the system since I went.
- In Arts & Science, after first year, you have to declare a Program of Study (POSt). That is, you have to choose your combination of Specialist, Major, and/or Minor. You can change this later, but it can get complicated if you need to take more courses to earn the prerequisites that the new POSt requires. For example, if you’re two years into an English POSt, and want to switch to Mathematics, you’ll have to take first-year math courses.
- As for changing single courses (not programs) such as PSY100 to SOC100, there are specific deadlines for dropping and adding courses. Drop/add before the course starts, and there are no penalties; but as time goes on, there are financial penalties and academic penalties (e.g. you can fail the course if you drop it too late).
- I don’t know what restrictions are placed on international students.
So, you can change things, but it’s not really easy. You have to plan ahead.
Each university has its own system, and if you are admitted to one, you should become very friendly with the Registrar’s office. They can help you navigate the system.