I booked SLC-ICN-GUM a few days ago. I was browsing the booking options and flight times and layover have changed 3-4 times in a week. I have a 12 hour layover in ICN now. I have to fly ICN- GUM on Korean and they keep changing flight times and aircraft. I was however able to choose my seats for all legs and actually upgraded Delta 1 on all flights even Korean with a GUC so it does work sometimes.