I think retroratchet is really useful. It could be implemented as assisted editing for the road editor, but I wouldn’t want it to go away.
Can I suggest making use of the information the user has already provided? Specifically, if they haven’t entered a goal total, you should shift the road up. If they have entered a goal total, you shouldn’t touch the total (see New UI date/total/rate pain point). In this case, if they’ve also entered a goal date, you should change the current segment (this is a hard deadline). If not (they’re using total and rate), shift the goal left.
EDIT: I’d advise adding some explanatory text to the advanced “Commit To” settings as well, saying that whichever fields you’ve set explicitly here, Beeminder will try to preserve them as much as makes sense across ratchets, derails, and other things that change your road.