I believe you need to do this in the Paratext Registry.
On the Basic tab, there is a Managing Organization field where the Administrator should be able to select another, or an additional organization.
The Project Administrator should be able to change the Managing Organization (Rightsholder) on the Publishing tab of the project’s record in the Paratext Registry, Rightsholder field. If this project has been submitted to the Digital Bible Library, then the change will need to be made by your DBL curator in the DBL.