Answered question
I subscribed earlier with fizz so I have access to their low price and it would stay that way even after the stabilisation period is over.
My question is : if I change my coverage for canada/Us for three months and get back to normal afterward, am I gonna lose my low price?
From this page
https://fizz.ca/en/faq-stabilization-period
If your plan was activated before April 12, it will return to its launch (introductory) price once the stabilization period is over. Please note, however, that if you change your plan after the stabilization period, regular prices will apply. You will be notified of the end of the stabilization period one month in advance.
Other answers
You can change only during the stabilization period. If you change after the stabilization your are going to have the normal rate
I have the same understanding as Irene.
As Irene has shown, any changes during the stabilization period will cause you to stay with introductory pricing (as long as you joined before 12 April).