Class Changes
When Do I Unlock Class Changes?
You unlock class changes after you beat the Greater Warped One of the Beginning Abyss. For summonable heroes you unlock a weekly reseting shop for class change books for your summonable heroes. For your main character certain class changes are locked behind story progression.
Do I Keep My Skills From the Previous Class?
Yes, you keep all the active and passive skills that you've learned from that class. However, certain skills and passives can only be used by certain classes. For instance, the description of the passive skill Behind Cover ends with \<Knight-specific>. This means you need be the knight class to have this passive active. Or take the active skill Hiding whose description ends with <Thief-specific>. This means you have to be the thief class to use this skill.
What About Passives Like Passive HP Up (Fighter)? Do I Need To Be Fighter Class For This Passive?
No, you don't need to be the same class as passives that list a class in parantheses. These parantheses just indicate origin of where the passive came from (and allow inheriting multiple similar skills from different sources). Angled brackets in the skill description, such as \<Knight-specific> indicate that use of the skill is limited to that specific class.
Can I Class Change Back?
Your main character can freely change between classes that you have unlocked and activated in the Well of Mind. Allies on the other hand cannot freely change back to a previous class. You either need another class change book corresponding to the character's original class, or a special item called a class change request that lets you change back to a previous class. Again, yes, you have to obtain or buy another class change item to change a character back to a previous class, and for handbooks it's a different handbook than you used to change the first time (the name of the book tells you the class you can use it to change to. Class change request items are given for free during class change events and purchased from certain limited packs from the jeweler.
Why Am I Level 1? Does My Other Class Keep Their Levels?
Levels are independent to each class so you are going to have to relevel.
Why Are Some Characters Missing A Class Change?
Anonymous units such as Human-pri can not class change. As of now newer released units don't come with their class change. Drecom will announce when their class change will come.
When Should I Class Change?
For your main character, class change as soon as possible. The Wanderer Class doesn't get any new skills after level 20. For your other party members, it's up to personal preference. However, with new level caps coming, this means new potential skills so you might as well level the class to max before changing classes.
Should I Stay As The Class That I Changed To Or Swap Back After Getting The Skills And Passives?
It depends. This is your personal preference for your playstyle. Keep in mind that classes scale differently on their base stats, have different resource pools, and having differing things they can equip.