Remove Node Reference
If I'm clear, you don't actually want to remove any of the press offices because existing press releases should still be able to reference them, you just want to remove the option to associate new press releases with certain press offices (while at the same time, creating some new press offices that can be referenced).

So really, we just want to filter the list of press offices that can be referenced by new press releases.

I would suggest that you add a new boolean field to the press offices, something like: "Can be referenced". Then create a view that filters press offices on that field. Finally, configure your Entity Reference field Mode to use "Views: Filter by an entity reference view" and select the view you just created.