I tried to rebook on FR webpage. If I change the date only to beyond 14 days af original travel date, an up-fare is indicated. The same applies if I try to change to different route. If I change the date to within 14 days, the "Free" change is indicated.

The odd thing is that if I try to rebook to a different route with a travel date less than 7 days from today (but still to later than 14 days before original travel date, i.e. within the +/- 14 days window) an up-fare is also shown, indicating that rebooking needs to be done at least 7 days before the new travel date, i.e. you can't wait to rebook until shortly before your new travel date - however, this is not mentioned anywhere.