I have an I ticket booked SYD-HKG-SGN. Due to a 7 hour layover in HKG, I want to change to an earlier flight HKG-SGN. That flight has plenty (9) of I availability.
Unfortunately SYD-HKG-SGN has 0 I availability - so can’t change without a change in fare class.
(We are also booked to return but from HAN-HKG-SYD; not sure if that would have bearing on the first leg (suspect not).)
Anyway, just adding a data point - even though our scenarios aren’t the same.