I have made changes to flights once travel has begun, within 18 hours of the 2nd flight.
E.g. original flight was DAY-ORD-FRA-PVG (UA/LH/LH) with an overnight (23 hour layover in ORD)
Once I got to the hotel in ORD, I was able to call UA and change the remainder of the reservation to ORD-ZRH-PVG (LX/LX)
YMMV