When you can use a different license?
To quote M. Warner Losh (with permission):
So long as there’s old code, you can’t replace the copyright, unlessthe old code that remains is not protectable by copyright any more(eg, if it is just { and } lines).If you add your own implementation of the 50 lines in question, or move them to a separate file, then you can put your own copyright/license statement on the code you wrote.