Trying to make sure I understand this chart correctly. I am currently in A/R on an upcoming RDU - LHR trip and am looking to move to one day earlier for my return flight (which is booked in R). PZ9 on the flights I'm looking at so MUA should clear immediately.

When I go to change my ticket to one day earlier, its a +$0 add/collect to go back to Economy in a U fare (not surprising since my original ticket is in A outbound and R return) and then it's a $50 add/collect to change to the earlier flight into the same R fare.

But then I go to look at the MUA chart and it seems to suggest that it's actually a co-pay (with the same mileage) on the U fare than on the R fare? Is that right? So I should just change to the U fare with the $0 add/collect since I'll be able to then clear the MUA immediately up to Polaris based on the PZ9? I'm confused -- obviously I want to save money on the co-pay so why wouldn't I do that but it just seems counterintuitive. I'm in the midst of a Platinum challenge so don't have any PlusPoints to use for a PP to Polaris upgrade.