Currently, there is no direct method to merge two separate Microsoft 365 accounts (one free and one paid) into a single account. However, you can manage your accounts by adding a personal Microsoft account to your work or school account, which allows you to maintain access to your learning history and credentials. If you need to transfer files from your free account to your paid account, you can manually download the files from OneDrive associated with the free account and then upload them to the OneDrive of your paid account.
For more detailed guidance on managing your Microsoft Learn profile and accounts, you can refer to the resources provided by Microsoft.