Hi
actually you dont need to change visa from work to that of student all you have to do is ask your employer if he/she will allow you to study and thats it. but if you have other things in mind than that you have to get a student visa anytime you want if youll get an admission letter from any university youre planning to get your degree and if you have all the means to support yourself (that is pay for your tuition fees and cost of living while youre studying). if you have all of these things there shouldnt be any problems for you to apply for a student visa.
Not quite, the OP requires study permit to study full time in a course longer than 6 months. S/he will be unable to hold a study permit and a work permit at the same time. (There are PT work permits for students off-campus, but only after completing 6 months of study)
PMM