You can do that or go to the new carrier and have them port over your number and automatically it will give Rogers the 30 days BUT the line will cancel as of that moment once the number has been transferred to the new carrier.
Your final bill will be ur owing Flextab + monthly fee + $12.50 disconnecting fee.
If you were looking to switch carriers, I would definitly call in an let them know so that they can see what can be done as well as provide you with all the infromation that you would need to know prior to moving your services to a different carrier. This way there will not be any huge surprises.
Just make sure to ask lots of questions. It never hurts to have the information.
Hope this helps. 🙂
And yes, the final bill would be your partial charges/credits for the month (depending on when you cancel your services) +Flextab + 12.50$. Meowmix is correct.