In the case of a single author, the copyright holder has all the same rights he/she used to have (including to re-license under a proprietary or different open source license), except the right to revoke the open-source license already granted to people who have obtained the source.

In the case of multiple authors, the legal rights of any one contributor are significantly curtailed. Unless they vote with unanimity, the license cannot easily be changed: contributor X has licensed her code to contributors Y and Z (and everyone else) under BSD licensing terms, and it cannot be relicensed without her consent unless her contribution is completely excised from the project.

Even then, the waters can become murky because most code cannot just be deleted from a project, other code grows organically into and around it, so the line between what constitutes a "derivative work" and what doesn't can become really blurry.