Answered question
Can the SIM card on an Account be deactivated and then activated immediately by another Fizz member?
Also, can the deactivated SIM card be re-activated on the original account? If so, how long of a dormant gap can there be?
No, you need to wait that the previous plan is closed, therefore, you need to wait the end of billing cycle.
Here you are
https://fizz.ca/en/faq/unsubscribe
After you close your plan you can give your SIM card to a friend. They can use it to activate their own Fizz plan (with their own settings, etc.). This will save them from having to buy their own SIM card.
Other answers
If your number has been transferred to a new provider, I believe someone else can re-use (re-activate) it immediately.