I have a summoner in Pathfinder. I have just reached level 5, and the PRD states:
Upon reaching 5th level, and at every third summoner level thereafter (8th, 11th, and so on), a summoner can choose to learn a new spell in place of one he already knows. In effect, the summoner “loses” the old spell in exchange for the new one. The new spell's level must be the same as that of the spell being exchanged, and it must be at least one level lower than the highest-level summoner spell he can cast. A summoner may swap out only a single spell at any given level and must choose whether or not to swap the spell at the same time that he gains new spells known for the level.
Emphasis is mine, other spontaneous spellcasters have the same rule.
When they say "at any given level", does it mean that at 5th level I can swap only one spell? Or does it mean that I can swap one level 0th spell and one 1st level spell?