With only 8 more A332's on the way until the A350s appear then I would hope US would defer retiring any 767s. Why put itself under any unnecessary fleet inflexibility or competitive short-term disadvantage?
Whether any suitable ETOPS 757s with RR engines are even on the market I have no idea. I've wondered before if the five 767ERs that UA/CO is retiring this year would even be available - or even worthwhile - for US to acquire in the event of needing to surrender any existing 757s.
Skycubbie