Ofcourse the contributors have signed copyright assignment to you (that's mostly standard practice) but they have done so for a project that at the time was wit

Ofcourse the contributors have signed copyright assignment to you (that's mostly standard practice) but they have done so for a project that at the time was with the liberal license. It may not be illegal but it's certainly abusive to then switch licenses because chances are those would not have signed the copyright away for free for a commercial product in the first place.