Understand how and when tickets change the lowest available fare class
Hi.
Is there a way to understand when an airline will change its lowest fare class offering? Is it based on ticket available or timing or both? For example, right now DL shows L and U fares for a flight 9 days in advance. Is this based on availability of tickets assigned to those fare classes (eg "There are only 2 tickets available at this price" per Delta.com) and then the price goes up and the fare class increase?
By the way, I have to book JFK-SFO-JFK. Leaving on a Monday (anytime) and returning on Saturday (probably the 11:30am flight). At Gold, I've accepted that I pretty much have no chance for an upgrade on Monday and maybe a better shot on Saturday. I need to book within the next two days (7 day window), and I am just wondering if the fare class won't change. I am guessing there is a system. Thanks in advance.