I am new to the guest upgrade and getting very confused. Most of the flights I am looking at have an unlighted F sign meaning that the first class upgrade is unavailable even if there are lots of empty F seats. However, when I added the flight to my cart, it says “You will be able to apply MVP Gold Guest Upgrade codes right after your purchase”. Does it mean I can still add the upgrade code to my friends’ itinerary and make them waitlisted for first class instead of getting confirmed instantly? I upgraded my friends on the flights marked with a lighted F sign and they got a confirmed upgrade instantly, their trip is far away from the 120-hour upgrade window. So I guess flights with a lighted F sign mean instantly confirmed upgrade while an unlighted F sign means waitlisted upgrade. Can anyone experienced with guest upgrades confirm this?

Also, if my friends change any segment of flights, will they be removed from upgrade and my guest upgrade passes used on their flights be revoked?