Suppose my 10th level wizard wakes up from a good night's rest and, using all 20 points of his intelligence, he realizes that he needs to change his spell list a little. It's time to retire Magic Missile, and prepare Fireball instead. The other 14 spells (with a combined spell level total of, say, 35) are not changed.

That is, he is changing one of his 15 prepared spells; the new spell is a 3rd level spell. He is not changing any of his other spells.

My question is, how long does this take? The rules in PHB are a little vague. Page 114 (emphasis mine):

> You can change your list of prepared spells when you finish a long rest. Preparing a new list of wizard spells requires time spent studying your spellbook and memorizing the incantations and gestures you must make to cast the spell: **at least 1 minute per spell level for each spell on your list.**

Is the wizard required to spend the full 38 minutes as if he was preparing an entirely new spell list, or only 3 minutes for the new 3rd level spell?