Have an economy class Award Ticket
BOS-CDG-FCO
Layover in CDG arriving 6:10 AM and leaving at 9:40 AM to FCO.

My ticket terms clearly state 50 euro change fee + any fare difference ; same for before or after first departure.
If I wanted to take a later but still same day flight to FCO (Rome) - AF has multiple per day flights.
Is this allowed paying the 50 euro fee only if the current list pricing is higher on the flight they already have me booked on CDG-FCO?
The 9:40 flight CDG-FCO is the most expensive flight listed for this day.
or is the fare difference based on what it would be if cancelling my entire trip and rebooking (in that case Award tickets are much higher priced today than when I had booked months back)
If I wanted to take the FCO flight the next day ; would that be different?

If only the 50 Euro fee per person ; ideally I would actually not do anything until landing in CDG and then decide if I want to stay for more hours in CDG at that time. This may not be an option.
A little more detail:
Flight arrives into CDG at 6:10 AM ; FCO flight 3 hr, 30 min later at 9:40
Will have our young daughter with us and not sure how she will be feeling. May be worth the change fee to us if she is not feeling well after the overnight flight.