PSA - Sales Order to Project Contract

I am evaluating PSA in Dynamics 365 Online.

I have followed all the tutorials and as far as I understand the Project Contract is just an extension to the Order table.

If I have a Project Contract, I select the drop down box to change it to a regular sales order.

The form flashes a few times and then reverts back.

The same happens if I go from a regular Sales Order to a Project Contract.

If these are the same table, should I not be able to convert the order?

What steps would have to happen for this to happen.

Thanks