Yes this is the rule
Any changes you make to your Home Internet plan will take effect at the start of your next payment cycle.
You can contact the support and try with them
To contact the support you need to go to this page https://fizz.ca/en/contact-us, Fizz push you to find your solution, therefore, you need to search in the solution hub to your solution and only after 10 seconds that you are in a solution page Fizz will give you the chat bubble.
If the chat bubble does not appear, this means that the number of sessions waiting for our answers surpasses our processing capacity. If that happens, send us a private message via Facebook Messenger or Twitter.
Your can also send a message via facebook messanger https://facebook.com/fizzca and click send a message (this is a bit slower)
You can also fill a contact form from your account Once logged in, from the account panel go to My Profile → Contact Form (the subject are limited) and fill out the form and someone will email you back.