CF: Spell Repertoire / Known Spells
You have a collection of spells you know by heart, have memorized, or call upon from deep within yourself. These were classically known as 'Known Spells', but for you this is called your Spell Repertoire.
Your Spell Repertoire is a pool of spells you have equal half your class level (rounded up), + your class's spellcasting attribute (Intelligence for Maguses, Charsima or Intelligence for Sorcerers, etc), with a minimum of 2. Each time this number increases, you learn of a new spell of your choice from the spell list(s) that assigned this common feature.
At Caster level 1, you learn the bare minimum amount of spells (at least 2).
Whenever you level up your character, you may exchange any 1 spell in you added to your Repertoire for another, as long as it is on one of the spells lists from the class that assigned this common feature. Certain features may add spells to your Repertoire, using language like 'x spell is added to your Library or Repertoire' (this does not include things that say 'you learn...' or 'you gain...'). These spells do not count against the maximum number of spells you know.
Once per day, after finishing a Long Rest or during your daily preparations, you may exchange one spell from your Repertoire with another from your spell list(s), as long as you are eligible for both.