We booked cruise buy 1 get one free airfare. Received flight arrangements yesterday which is 30 days out. Outbound flight was perfect. Went on Alaska air to upgrade our seats. Paid the appropriate upgrade fee. Return flight is a totally different story. Cruise docks in Vancouver at 6 AM. NCL booked us on a Air Canada flight at 11:20 pm arriving at Chicago O`hare at 5:30 AM. There is a Air Canada flight at 1:45 PM that has availability. NCL and Air Canada won`t change us to earlier flight
And each airline has their own rules. There is no blanket "number" that qualifies as "major".
Plus, as noted, the specific fare rules for your ticket are part of the governing structure for your travel.