Sorry but it is not possible, this is the communication
To thank you for being true ambassadors from the get-go, the price you are paying now will stay the same (1). If you change your plan, however, you will lose the price you are paying now, and regular prices will apply.
When you subscribed to Fizz during the stabilization period, you were informed that your plan was to switch to its regular price once that period was over. But to thank you for your patience and collaboration, you get to keep the price you are paying now until early 2020. You will be notified of the switch to the regular price 30 days in advance. If you change your mobile plan, however, you will lose the price you’re paying now, and regular prices will apply.
You can port your number when you are activating a new plan.
Currently it's not possible to port your number to an active Fizz plan. I read that Fizz may change this in the future, but for now it's not possible.
If you want to change your plan or register for a new one, you have to pick from the plans/pricing offered currently.
My suggestion is to keep your current Fizz plan. If you can, port your other number elsewhere for now. Or consider giving it up.