If the copyright has always belonged to the employer then OP never had the legal ability to license it as open source in the first place. The employer could pre

If the copyright has always belonged to the employer then OP never had the legal ability to license it as open source in the first place. The employer could presumably shut the whole project down and in theory even go after others for copyright infringement retrospectively. If there have been other contributors then the employer would not magically get the copyright to those contributions though, which could leave the whole project in limbo with no-one having enough rights to continue working on it and releasing the results.