My understanding is that the copyright owner may do whatever they please, including selling modified versions with their own custom proprietary changes, or maintaining multiple versions under differing licenses. However, it's true that once they've released the code under an open license, anyone who has saved a copy of the code may release their own forked version in the future, regardless of what the copyright owner does with their version, so long as the forker continues to follow the terms of that license for the code still under copyright of the original owner. New code under the forker's copyright may be released under other licenses if the forker chooses, etc, etc.