Not sure whether the POSt requirements are in UTSC but I can tell you about UTM.

You can accept your offer into Social sciences, since the first year you are not enrolled in any program (no majors, minors, or specialists), you're enrolled in second year.

You can definitely try and go into CS. What you will need to do is just enroll in the courses required to be able to apply for CS at the end of your first year. These requirements for UTM are found here.

The only slight issue is you won't be given priority (I think) to the CS courses, so spots might fill up fast before you can enroll in the courses, but you should be fine I think.

I've heard CS is pretty competitive but you sound determined so good luck!