Specifically, the GPLv2. The creator of a work can allow people to upgrade to a higher GPL version:
This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.
If I modify a work with a license statement such as this, am I allowed to say that people who then further modify my distribution cannot upgrade it to a higher GPL version? Or do I have to allow them just as the original does?