Question about promoting & retaining skills

Hi everyone,

I started playing Dark Deity 2 and digging it a lot.

I have one question about the class progression / promotion though:

Let's say I promote Cassandra to Elementalist. Can I "master" the skilms of that class, switch to Juggernaut, learn those skills and so on...?

Like can I "master" or permanently keep all tier 2 skills before moving on to tier 3?

Thanks!