I think the workaround I described would be much easier to implement i.e. a user could toggle an option named Show rewrite button near Save in its parameters (same place as where he setup his password and language).
It that case the user would spare one click if he wants to replace the original question. Having this button would mean that the Save button return a pane similar to when we save a question for the first time.
I may suggest something very specific to my needs, but if this rewriting could fit in an issue I'm ready to do some mockups or deeper explanation for it.