You are not really allowed to hold both permits.
While I was in college, I had applied for a job on campus and was hired. I took the employment letter to the CIC office on St Clair and another study permit was printed and given to me.....the only difference was that the new study permit indicates that I am allowed to work while studying at the campus and cannot work for no other employer besides that.
I am not sure about having a work permit and then trying to apply for a study permit. I would believe that a new clause will be added unto to your current work permit but I doubt if you will be granted both work and study permits.
