Gaining a level won't remove abilities that a Class had at a previous level.

Some Class cards have an effect that increases when more are under your control. For example, if you have multiple Barbarian Class cards, you roll that many additional dice and ignore that many of the lowest rolls.

Each Class starts with only the first of three class abilities. As the first level ability resolves, the Class becomes level 2 and gains the second class ability. As the second level ability resolves, the Class becomes level 3 and gains the third class ability.

If you have more than one Monk Class on the battlefield, their abilities are cumulative. For example, if you control two Monk Classes, the second spell you cast each turn costs less to cast.

If Monk Class is the first spell you cast, the next spell is the second spell and costs less. If Monk Class is the second (or later) spell you cast, its bonus won't apply until the next turn.

You can't activate the first level ability of a Class unless that Class is level 1. Similarly, you can't activate the second level ability of a Class unless that Class is level 2.

You can multiclass or even control multiple Class enchantments of the same class. Each Class permanent tracks its own level separately.

Each Class has five abilities. The three in the major sections of its text box are class abilities. Class abilities can be static, activated, or triggered abilities. The other two are level abilities, one activated ability to advance the Class to level 2 and another to advance the Class to level 3.

Gaining a level is a normal activated ability. It uses the stack and can be responded to.