Question: Overcoming the barrier to understand Dom 3?

[Yskonyn]

For the sake of argument here's the link to the guide I found fascinating; Why Caelum should go for Blood magic

Another thing I notice, is that you are talking about 3 to 4 levels of each magic path.
Can levels be increased during gameplay? Because I always thought once you have a certain level chosen, you cannot increase it anymore and thus the maximum level spells are restricted to your initial build.
Am I mistaken?