I plan on changing one of my courses, but I want to fill out the payment agreement so my status says Fully Registered. If I fill out the payment agreement now and verify it, can I make changes if my courses change?

Yup you definitely can make changes, you'll just have to confirm your payment agreement again once you make the changes.

Alright, and under the plans, there's a whole bunch of payment options. I've applied for OSAP, and and depending on that to pay as much of the cost as I can. The OSAP Flex plan is probably what I have to choose, right?

If you can pay in full then I would say do that because then you get the payment out of the way and you don't have to think about interest costs and in January all of the OSAP cheque will go to you (as long as you don't owe the school for anything else). I would use the estimate that OSAP gave to you when you applied for OSAP and assess whether or not you can pay in full. From my experience the estimate is usually pretty close, I've even gotten more than the estimate before, but budget for around the estimate.
If you can't pay in full then yes I think the OSAP Flex plan is probably your best choice. It seems to be designed with OSAP in mind.

yeah thats what i'll do. thanks a lot

Anytime!

I've found, for myself, that paying more than one installment with OSAP (i.e. in September and then January) works out better. Since I'm getting roughly 6k in September from OSAP, and my tuition is 6k, then I'm left with barely any money. If I pay in installments, I think last year I had to pay 3500 in September, then I have money left over for rent & food. But I mean, it all depends on your situation. Just think about what's good for you.