Need to approve invoices created before AME was setup

Hi,

After setting up a new company, Invoices were entered and accounted before setting up AME Approval.

Now we have set up AME approval but Initiate Approval is still greyed out on those invoices!

We enabled Force Approval to approve them but Force Approval option in AP Invoice > Actions is also greyed out with the Initiate Approval.

We have created a new invoice just to see if the approval is working for newly created invoices and yes it is.

How can we approve the invoices that were created before AME was setup?

Thank you,

Christine