Is it possible to undo mirror on a model to only work on one side without having it done on the other side and come back to mirror without having to box cut the model to mirror again. I only want to work on one side of the model for the time been, but want to have mirror again.

The best thing to do is click the little ‘eye’ in the modifier. This means that the mirror is still there, but will not be displayed. The modifier will still be active, so any modifications will be mirrored.

Does that mean you want to break the symmetry?

No, I just want it temporarily disable so that I can only work on one side.