U class vs. I class?
 

Just a few questions from a newbie. Sorry if this is in the wrong forum.

I bought a ticket a month ago for $300. The price went down to $267 today. I called continental and asked if I could have the price adjusted. The woman I spoke to said that because I booked a U class ticket and the ticket on sale was an I class ticket, I would have to pay the $100 change fee.

What do each of the classes mean? How do classes change?

Thanks so much!