I've done the same thing when I got here. Got here on an extendable tourist VISA with letters indicating that I'm a student, and so changing to resident VISA and then to ARC was a breeze. If you don't have those letters on your VISA application, what you need to make sure is that when you apply for the VISA you tell them this is what you're planning to do so that the VISA status would be changeable. Supporting documents from your university might help. especially a letter from the university you're going to.

I think that when it comes to a visitor's VISA this means it's extendable (don't have "no extensions" mark) and that you get enough time for you to complete the procedure (pay the school, get receipt, go the government office nearby and do the procedure).

Good luck, would be nice if you'd tell us how it ended up for you when you're through.

Fili