I was able to change dates, routing (HKG-EWR-ORD to HKG-NRT-ORD), and carrier (United before, *A Partner now) for one of my segments with no additional miles charged (same origin/destination). Also, I changed the date & time (same O/D and carrier) for an intra-Asia segment. Just cost me like $10 in extra taxes. ^ Did it on the phone as online kept erroring out on me when I attempted to search for new flights after clicking Change Flights.