Garrett pretty much said it all. You have to write a letter to the department you are interested in and describe why you want to switch streams. Mechatronics is not a 4.0 program, so you might need a slightly higher than a 4 this year (not sure).
Ned Nedialkov is the associate chair of the CAS department, email him and describe your situation and ask him what average you need in order to switch to tron. He should be able to help you out.
I am basing this on a similar situation that happened to my friend. He wanted to switch streams and emailed the associate chair and then he got back at my friend and told him the procedure and what average he needed to get.
Good luck with everything!