How do I transfer my old FIZZ phone number?

Fizz guarantee the number for 60 days, after that who knows

Did you try to call the number?

this is the guide
https://fizz.ca/en/faq/reactivate

How to reactivate your Fizz account when you have a SIM card:
1- Go to the Reactivate page and log into your account.
2- Review or adjust your plan details and click Confirm.
3- Verify your phone number (if you are reactivating within 60 days) or select a new phone number. Learn more about changing your phone number
4- Review your addresses and plan details, then click Submit.
5- You go to the payment interface to pay for the first month of your new plan.

You can contact Fizz support by chat, from 8 AM to 9 PM everyday:

1- Connect to your Fizz account by clicking « Login » in the top right corner at fizz.ca
2- Click on this link: https://fizz.ca/en/faq/reactivate
3- The green chat bubble should appear after a few seconds in the bottom right corner.

Edwin,
Fizz can keep your phone number for 60 days only.
You can re-activate your Fizz sim card but you will get another phone number.
Please, follow Mike's recommandations.