Ken, if i'm thinking what you are thinking, it's no good. Assuming that you purchase a Q fare and see availability in C on the return (say, 7 days from return), you will not meet the minimum purchase requirements on the "changed" fare basis and will likely have to buy full fare to be eligible on the u/g. I've tried it before after getting a break on a business trip, and AC wanted to see my fare go from a Q to a Y - no B, H or V even though they were available.

What about the other way around? In certain circumstances, what if you purchased an H fare and saw that 7 days out of the return you were out of luck on the u/g (zero C). Could you change your fare basis (paying the change fee) and get a credit on a portion of the ticket? In many cases, the difference between the change fee and the next lower fare basis is signficant.