Magery advantage (buy / increase)

[GURPZine]

Is it possible to acquire/buy the Magery advantage or even level it up after character creation? It's not clear to me. I've always understood that after character creation (so throughout the adventures and campaigns) it would not be possible to acquire this advantage or even improve it if I already have it, because it seems to me to be an innate gift. I thought it would only be possible through divine grant, powerful rituals, extreme situations.