Hi Adrian
I am not 100% sure but I faced this similar issue long back where I was not TC for the licenced node but was TC of the free node app that we created. I think we deleted the sandbox and then the TC of the licenced node was able to select that app from the list as he was also team member of the free node app.
This how to can help you unlink / delete the free node.
If any issues you can file a support ticket with Mendix.
Hope this helps!
Goto https://sprintr.home.mendix.com/ and select nodes from the left menu
Click the Select project button from your paid node and select the free node project from the list.
This should connect the paid node to the free node.
If this is not possible contact mendix support