UA385 -- EWR-DBV -- on July 3, I see 11 open J seats and 10 open PE seats on the seatmap -- maybe they processed a few upgrades. The inventory is J9 .. P9 PN0, indicating that while they're not giving any upgrades at the moment, they're also not confident that they can sell the remaining seats. DBV isn't a huge business destination so that probably factors in somewhere.

It's also O9 ON9 A9 R7 RN0, so I think you'd likely end up in R with a buy-up. But I'm still hesitant to guarantee that your plan will work. At a minimum, I'd check the price to do a flight change into PE vs. the buy-up cost. Do you have PlusPoints applied at the moment? If so, what's your spot in line?