So, my understanding with the limited information (and admittedly limited looking into it I've done) is that your Main Class is fixed, and you can only change your secondary class which augments your main class.
I personally, kind of dislike this. I would like to see a way, even if time consuming and/or costly (for ingame resources) of changing that.
If I were to pick, hypothetically, Fighter, and found after spending time levelling, making friends, etc, that it's not working out no matter the secondary I chose, for some reason, be it playstyles not feeling right, it not suiting me, some changes to the class via patches or I simply struggle due to any ping issues, I would not want to lose anything I spent money on, or my name, or anything, while wanting to try something different.
I understand that they kind of want your decision to have consequences, however I am most likely not alone in wanting to have nearly everything I do in an MMO tied to 1 name, 1 character. I also understand not wanting a system where you can change class at a whim, such as FF14, Archeage or PSO2, as those games can result in people swapping back and forth on a daily basis, or even more often, depending on what task they wish to accomplish, and can result in metas where people expect you to be able to play a class you're not at all. That's why I mentioned this being a time intensive, or costly endeavour.
Perhaps a questline which takes multiple hours, or doing tedious tasks (For example a mindnumbing "Kill 1000 of X enemy") in order to change your primary class. It would help there be a bit of a compromise between these. Because, to be honest, were I to pick Fighter, and then some patch changes tweaked the balance and I no longer enjoyed it no matter my subclass, I'd be more likely to quit than spend time investing in a whole new character. I invest in 1 character, not multiple. Always have done, always will. I am probably not alone in that feeling.
All that said, maybe there's differing perspectives, maybe there's things I haven't considered about going such a route, so I'd be interested to know what people thought of it, and maybe even have it be considered by Intrepid
