In addition, what you can try to do if you want some kind of certainty before calling AC is to price out your exact itinerary using ITA Matrix, one with AC codeshare flight number and one with LH flight number for the same segment and see if the prices are the same. If the prices are the same, chances are it's possible to swap AC flight number out for LH. However, if your itinerary is one way across the pond, it is very unlikely AC will swap the flight number given the ticket was issued on 014 stock.