Welcome to the community!
If you purchase a phone with Fido's Payment Program, you will be able to change your plan to one in the same category. That is, you could switch from a Data, Talk & Text to another plan with Data, Talk & Text. On the other hand, if you wished to switch to a different plan category (ie Data, Talk & Text to Talk & Text), you will be charged the outstanding balance of your Payment Program.
Hope this helps 😀