Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Hello
If you go from an annual plan to a monthly plan, it is usually treated as a separate plan. So you might end up paying for two separate subscriptions, an annual personal plan and a monthly premium plan.
Check your account https://account.live.com > Subscriptions, how many is listed there? One or two?
If the personal plan is no longer listed, then you should be all set, as it will do a conversation
If you have two separate plans listed, then you might cancel one of them, and discuss with support to see what refund option you have. Typically you can only get a refund by canceling within 30 days, but if this is made by mistake, explain to support, see if they will make an exception for you.
Go to https://support.microsoft.com/contactus, Type "Billing" and press Enter. On the next screen, click "Contact support" at the bottom of the page, then choose "Microsoft 365" > "Manage my subscription". You’ll then see the option to chat with a Microsoft support agent who has access to your account to help review the issue.
You can reference this detail step by step instruction with screenshots I posted in this thread: https://learn.microsoft.com/en-us/answers/c/2714817