Hi @rainbow_guy ,
if you "upgrade"/change from Azure AD Premium P1 to Azure Azure AD Premium P2 you will pay the price of P2 and stopped paying the price for P1 after the change got active.
You don't pay P1 and P2 in addition. You will pay just the plan you selected and which is active.
Based on the price information here you will pay 9 US$ instead of 6 US$ per user per month if you switch from P1 to P2:
Source: https://www.microsoft.com/en-us/security/business/microsoft-entra-pricing
(If the reply was helpful please don't forget to upvote and/or accept as answer, thank you)
Regards
Andreas Baumgarten