Originally booked roundtrip U fare code ORD-MSY. Upfared to A class on both legs. tried to SCD to ORD-IAH-MSY. new flight was A fare ORD-IAH, and only economy (B) from IAH-MSY. Thought I could convince the agent to let me take the economy seat (B). She said that after an upfare, the original fare code is relevant and it was expensive to switch from the original U fare to B, even though I had upfared to A. Was going to cost $1k, so did not make the change. she also said in irregular ops I would be subject to original U fare code. agent seemed knowledgeable and as if she had many years of experience. any thoughts on if she was correct? no reason for me to think she was wrong.