Greyed out stands - SAN Diego

So, I have stands that are unlocked for say small planes, but the option to upgrade them to M, L or X doesnt seem an option - yet M,L,X are there just greyed out. Is there a way I can upgrade these stands so t that they accept planes of different sizes?

There are certain stands that due to their positioning can only allow S aircraft. So if they are greyed out, upgrading is not possible. This would mirror real life usage of the stands as some of them are very tight even with S aircraft, i am thinking specifically of 1,2,3, 19 and 20, you will find some of these at most of the airports in the game.

if an aircraft is cleared to land and is planned to use that gate, it cannot be upgraded until after that aircraft departs again. Once that aircraft departs, and before you clear another aircraft to take its place, you should be able to upgrade once the spot is vacated.