Hi Damien,
If your problem can be solved with moving one whole project from one account to the other, then we are currently building out support for this on our end. In this case, please contact our support team and we can help you once that is built out (which should be over the next couple of weeks). If it is about migrating individual customers and/or apps between projects, for now you will have to go the route mentioned in the above article (since that is a much trickier problem to solve, especially since there may be conflicts between origin app and destination app).