Just had a call with the customer service, finally someone tried (and that's the word) to explain:

Apparently with booking an "O" class when you try to change the ticket very close to the new ticket date it will always book into the highest class. "It's a restriction of your ticket type".
-> When asked if she can see "O" available to sell - yes, she can, but not to change.
-> When asked where this was in the fare rules - could not answer;
-> When asked for a supervisor - not available;
-> When asked if this rule applied 48h before the new flight? - "Not exactly"
-> When asked how would I know when I see "free flight change, only fare difference" on website that this means "only fare difference to whatever fare we decide, not what's available for selling"
-> When asked if I can get this in writing - not possible

Frankly dealing with airlines is getting more and more of a joke. I've 0 problem rules getting enforced when they apply but apparent random rules without the ability to "prove"? Just bullocks.
Imagine me calling them and saying I wanted to change into "J" because "that's a rule of my ticket" and them telling me "where is that written?" and me saying "It's not but it is" lol