I know it's been a while since the class store went down, but I've finally finished the other admins' work and opened it up in time for the holidays.
It works like this:
If you have three classes already, you can't add another (obviously) but you can swap one of your three classes for any other class.
If you have less than three classes, you have the option to swap or add more classes on until you reach that cap.
You can also remove a class any time.
This swapping system is kind of complicated and involves a lot of variables so it took some time to figure out and take off where the others left off,
plus real-life stuff gets in the way of how much time I can spend doing something like this, so I'm real sorry about the long wait.
A forewarning for some of you: not everyone's tags are in sync, so some things that should appear on the store for you might not be there. I can fix this if you let me know what's missing for you via PM, this thread, support thread, catching me in-game, mentioning it in the official discord - however you'd like.