The only way to convert from GPL to a different license is for all the copyright holders (approximately every contributor who has a single line of code in the project) to release a version with a new license. For projects with a small number of contributors this is simple, just call them up and negotiate a proprietary license. I expect that there would be better chances of success if the proposition was "money for license" instead of a plea to go to a BSD-like license. Of course the more contributors the greater the likelihood one of them will reject the offer and thereby make dual-licensing impossible.