Being devil's advocate here - have you tried changing ferry tickets last minute? I have the (dis)pleasure of having to travel by ferry quite a bit this year and find that change/cancellation policies are generally rather unclear/unpublished, and from what I read in some cases (When not travelling with an eticket) requires to actually return the physical ticket to a sales point prior to departure. It may - technically - be possible to make changes for little (or no) additional fees (Though its difficult to know due to there being very little published details..) but it still seems to be quite a pain it the behind!

For your AF case; I'm assuming that this would qualify for proper compensation as the CDG flight is over 3hrs