If they have a similar flight available, you won't be able to change locations. My advice is to have specific flights #s and times when you call them. Look up the options ahead of time that they will probably suggest, so that you can know ahead of time if you have a case against those working with your original itinerary. Then, you can suggest flying to Marseille as an option that could work for you.
For instance, we had a Delta flight cancelled months before our trip, but we needed to arrive in Zurich in the early part of the morning to ensure we could take the trains into Wengen and arrive by late afternoon. So, I had the specific flights I was willing to take when I called Delta, and turned down some of the initial options they offered that were arriving too late, and the one with just 60 minutes to make an international connection.