A new license legally becomes active once it has been made public (i.e. it is committed to the source tree) and is not retroactive (for legal reasons).
And because of this, unless the owner/maintainer is willing to nuke a huge chunk of the history within their public repository (with all repercussions which might come with it) it is still possible to make a safe fork of a open source / open core product suddenly moving into a different direction. Time travel to the point before the new license was added to the repository, and fork from that point in time.
The owner/maintainer involved might absolutely not like that but legally they have no choice to allow it, given the license and its permissions at that particular moment in time in the repo.