Resonance is a very weird stat in Mage Revised. The problem with this stat is, that for one thing it's not good explained mechanic-wise and it's kinda vague.
You can use Resonance (if you want to - there are many people who would argue against it) and it can change:
A mage's Resonance usually changes only when she's heavily influenced by strong magical powers. This change can occur during a Seeking,
after botching an Effect in a spectacular fashion or when visiting a strange Horizon Realm.
So you see, that it's purely up to the Storyteller. My take on it would be that a player could tell his GM that he wants his mage to emenate more of a specific emotion and wants to integrate that into his magic (since it's possible that similar Resonance can lower the difficulty for a Arete dice roll). The GM then thinks of a good way and they have a blast :)
Resonance is more of a roleplaying aspect than a mechanic/gaming one. Hope this helps.