I just looked up the itineraries for Costa neoRomantica, and noticed that the June 1 sailing from Amsterdam also allows embarkation in Le Havre on June 7. There's also a sailing that allows you to embark in Le Havre a week earlier, May 30. If you are able to change your sail date from May 26 to May 30 or June 7, perhaps Costa will pick up the fee to change the dates of your flights. If not, your travel insurance may.
Let us know how it goes.