Migrate Teams Resource accounts

as far as I know there is currently no way to convert a regular user account to a Resource account.

This is because Resource accounts have different properties and permissions than regular user accounts.

The best way to migrate your Teams Resource accounts is to recreate them in the new tenant. 

You can create a resource account in the Teams admin center.

1. Sign into the Teams admin center.
2. Expand Voice, and then select Resource accounts.
3. Select Add.
4. In the Add resource account pane, fill out Display name, Username, and the Resource account type. The resource account type can be either Auto attendant or Call queue, depending how you intend to use this resource account.
5. Select Save.

Once you have recreated and updated all of your Teams Resource accounts, you will be able to use them as you would any other Resource account.

Manage resource accounts for service numbers - Microsoft Teams | Microsoft Learn

I understand that this is not an ideal solution, but it is the only way to migrate your Teams Resource accounts at this time. I hope that Microsoft will add the ability to convert regular user accounts to Resource accounts in the future.