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If you've purchased a Y/B/M fare (for international) and there is inventory in the appropriate upgrade fare bucket (Z for DL, U for NW), then you can confirm the upgrade ahead of time.
RDU-JFK-LON would require 1 PMU (and another for the return). I would assume there is some limit to the number of segments you can have and still use a PMU but I don't know what it is.