No, to find the answer, fall back to copyright law. If you own the copyright to some body of code in entirety, you can license it under as many licenses as you

No, to find the answer, fall back to copyright law. If you own the copyright to some body of code in entirety, you can license it under as many licenses as you want. GPL for the community, proprietary trade-secret in exchange for money, and everything works fine. But.... if you take a pull request from someone, either they have to sign over copyright to their patch, or else you have to involve them in every relicensing negotiation, because now they are a copyright holder, too, in the work.