The traveler assignment can be modified even after the deal was finalized.
In order to do that, go to arrangement section into the deal:

Under Pax, you have the total number of travelers assigned per arrangement at the moment of creating the deal.  Drag your mouse over the number and you have the option to Open Traveler Assignments:

Click on the number of the travelers. The Traveler Assignments is now open and you can reassign travelers: