Best is to leave reservation as is.. with UA MP number on the main reservation.

Then usually prior to boarding, at the gate, for the flight you want credited to another FFP, you just whip out your FFP card and hand to agent and say to them to change your FFP to this one for this flight. Try to get them to reprint you a new BP.

That usually works. Make sure that the following flights still have your UA MP in it.