I have a huge amount of data left, will it rollover if I adjust my plan to 0 GB for the next month?

The data has a validity of 90 days, if you put your next plan to 0Gb you can still use the data till the expiration

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Does my rolled over data instantly expire when I change my plan?

Of course not. You keep your rolled over data for two (2) complete payment cycles even if you change your plan during that time, and even if that new plan has no data.

Source: https://fizz.ca/en/faq/what-is-rollover-and-how-does-it-work