Question
Can I detach a Revit file from the Central Model, so it becomes a standalone model and not workshared?
Answer
Yes you need to Click 
> Open > Project, browse to the central file or even the local file. Before you click open tick the box ‘Detach from Central’ at the bottom first then click Open. Revit will ask you do you want to preserve or discard worksets, just select the relevant one.