Relicensing isn't necessary. If you violate the GPL with respect to a work you automatically lose your license to that work.
It's enough if one or two main contributors assert their copyrights. Their contributions are so tangled with everything else after years of development that it can't meaningfully be separated away.
It's enough if one or two main contributors assert their copyrights. Their contributions are so tangled with everything else after years of development that it can't meaningfully be separated away.