下記リンクにある、この項目がトピ主さんに該当するのではないかと思うのですが…。
https://www.canada.ca/en/immigration-refugees-citizenship/services/study-canada/work/work-off-campus.html
If your study situation has changed
If you weren’t eligible to work off campus but now your study situation has changed, you may be able to change the conditions of your study permit.
You can apply if:
you’ve changed your study program to one that may allow you to work off campus and your study permit says “This permit does not permit the holder to engage in off-campus employment in Canada”, or
we included a condition on your study permit that may no longer apply
You must apply to change the conditions of your study permit. There is a fee to make this change. You must do this before you can apply for a SIN.