Hi there, @jepto.

Welcome to the Community. I'm here to help you get past the error that you received when changing an account detail type in QuickBooks Online.

When changing the account type of the accounts, you'll need to start it with the sub-accounts. If all of them have been changed, you can go ahead and change the parent account.

Here's how:

1. Select the Gear icon.
2. Under Your Company, select Chart of Accounts.
3. Locate the first sub-account to change.
4. From the Action column drop-down menu, select Edit.
5. In the Account dialog, clear the Is sub-account checkbox.
6. Select the new Account Type from the drop-down menu.
7. Select the appropriate Detail Type from the drop-down list.
8. Select Save and Close.
9. Select Yes in the prompt to change the account.

Repeat these steps for the remaining sub-accounts, and then change the account type of the parent account.

For more details about this process, please refer to this article: How to change the type of an existing parent account or sub-account. 

That should it. If you have any questions about this or anything else regarding QuickBooks, feel free to reach out to me here in the Community. Have a great week ahead!