Fare rules for changing return segment of flight (after flying first leg)
 

I'm looking at rebooking the return leg of a flight that I already flew the first leg for, and I'm trying to figure out how Delta assigns a new fare price.

The return leg isn't until next Tuesday so my main concern is, if I wait longer, will the price go higher? My assumption is that it's more based on the number of seats available in that specific fare class vs. a timing issue -- is that correct? I base this on rebooking these flights in the past and also looking at how prices for rebooking to tomorrow are actually cheaper than next week.

Can someone explain how this works?