what would happen if I change my plan now?

According to the information that has been released, if you change your plan during the Stabilization period, you will continue to get Stabilization pricing. If you activated prior to April 12, you will continue with the Introductory pricing when Stabilization ends. If you activated after April 12, you will go to regular pricing after Stabilization ends.

You'll lose the introductory prices if you change your plan going forward.

Mike and Goat are correct. Adam, the information you gave is incorrect.

According to https://fizz.ca/en/faq-stabilization-period

For members who subscribed BEFORE April 12, 2019.

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.

For members who subscribe ON, OR AFTER, April 12, 2019.

Your plan was activated on or after April 12? Once the stabilization period is over, regular prices will come into effect and will therefore apply to your plan. You’ll be happily surprised by these once the stabilization period has come to an end. You will be notified of the end of the stabilization period one month in advance.