A buddy of mine calls Flex real first class, because you can change your ticket with no cost, the other he refers to as "fake first class" I always take the cheaper one. It has only bit me once, I was is Vegas and decided to leave a few hours early, called Delta and asked if they had business class available and could I change my ticket. "Sure, no problem" I get to the airport and yes I can change my ticket at no charge to a center coach seat!! I hung out at the bar and kept my original seat and learned a valuable lesson!
I've found if you call and you're super nice they'll change it for free. I did have to pay a $200 change fee last month but they waived the fare difference which was around $800 (I bought my ticket early, needed to change to be on Grandma's just purchased flight).