Don't cancel with your old provider. You'll lose the phone number. You will need to get the account # to port your phone number, though.
You can actually do this in Self Serve, if you're registered for it. Log in and go to the Mobile Phone section, then click on the box to change your phone number. The instructions are there. Input your account # when prompted, and then submit it. It's usually pretty quick; anywhere from 2 mins to 2 hours. If it doesn't go through after a couple of hours, you should call Koodo to see if there's a conflict.