To have contributed under BSD means that you have contributed under a license that permits relicensing. To contribute under the "BSD license but secretly I'm not going to let you relicense" means that you have not in fact contributed under the BSD license in the first place, but your secret license.

If you want to contribute under your secret license, what it means is that you don't want to spend any time in the first place contributed to the BSD-licensed project. Make your changes and use them, but don't contribute them back. The BSD license permits this fully, including your ability to distribute and sell the resulting software, subject to the advertising clause as appropriate. So it's not like there's anything onerous about this idea.