I did not tried it myself yet but yes, from what I know it is prorata
From the manual:
You can modify your plan anytime during the period. The features of the new plan will be immediately accessible, and the amount will be prorated. You’ll be refunded for the number of days you haven’t used on the previous plan, and will be charged for the corresponding days on the new plan.
https://manual.bubble.io/account-plans-and-billing/plans-and-billing#modifying-a-plan