Indeed, being warned 5 months in advance your CDG-MIA flight will depart 3 hours earlier as expected should not be a big deal for most travellers. One arrives 3 hours early at the destination so any business planned there will be fine. The only problem can be appointments in Paris before the flight leaves, but with 5 months in advance at least in my (busy) job adjustments in my schedule are no big deal. And as all others already indicated, reducing a return flight to a one-way flight usually does not give any cost advantage but can double the ticket price or worse, certainly if you fly economy. So sticking to the ticket seems the simplest option. Or do you fly business? Like Goldorak indicates, it really would be helpful to understand why asked to be at the airport 3 hours earlier as initially expected with a 5 month notice is such a problem.