I'm looking at a TATL trip, and the R/T is now priced at about half or third of the usual amount. For that reason, O/W would be of no interest.
Right now the latest bookable date for the return leg is the end of April '21. The desired date is only 1-2 months beyond that. I understood from the above comment that the change supplement will be based on the difference in rates on the date of change. Let me try a couple options:
1). I make a change in a few months, and before the departure date -- e.g. in Aug '20. At that thime the R/T fare could easily be triple the current amount, and that would cancel any incentive for me to book now.
2). I make a change after completing the AAA-BBB leg -- e.g. in Jan'21. At that time UA will be comparing fares for the remaining O/W BBB-AAA between 2 dates -- the original in Apr'21 and desired in June'21. Those numbers would likely be very similar, and my supplemental payment correspondingly small or nil.

Am I now understanding the UA offer correctly, or still missing something?