Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Which steps did you follow, exactly? Note that you can't really downgrade a subscription. What you need to do is cancel the more expensive subscription and then order a new one. This is different from upgrading, which is possible, and it means that the time for the subscription will be extended. See https://support.microsoft.com/en-us/accounts-billing/subscriptions/switch-between-microsoft-365-subscriptions .