At the risk of sounding like a broken record, people miss the final segment of a trip all the time. Recall that all carriers are used to dealing with the, er, lowest common denominator of pax sophistication. Carrying tens of thousands of pax each day, each and every day UA sees people get lost and can't find the gate for the last segment, fall asleep in the terminal and miss the last segment, get drunk in the bar and miss the last segment, etc etc, every single day. They are not going to "track you down" or "revoke your MP status" -- they'd have to employ several full time people to do it! The fact that your SFO-LHR record might overlap slightly in time with your last segment of the old itin is never going to be noted, and even if it were, the easiest assumption is it's a mistake of no consequence.

And as others have said, your bag tag ex-SYD can say SFO, DEN, or Timbuktu -- it doesn't matter -- the bag is in your hand at SFO and there's nothing anyone can do about it.