Yes but only to an extent. You will need work on all the final revisions that a contest holder might ask for during the handover stage as long as they are in tune with the selected design. But if there are going to be any major changes, then you might just ask for a reasonable price.
The best bet is to talk to the contest holders directly for the changes using the “Activity” tab in the Design Handover stage.
Note: Most designers are very cooperating to make minimal changes to their winning designs.