In the section Preparing and Casting Spells (PHB 114) it is said (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 spell book 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.
The first highlighted segment makes it so that a wizard is essentially preparing their spells for the day (or a very large portion of the day with a good nap). However, the second highlighted segment indicates that the preparation for each individual spell is not that onerous (6 first level spells would take less than 10 minutes).
The wizard in my group has realized that he would like to have two sets of spells, one when combat was unlikely (5 non-combat spells and 1 combat spell) and one when combat was likely (5 combat spells and 1 non-combat spell). Sometimes, the nature of role playing makes this impossible. However, when the party walks up to the mouth of the cave or the entrance to the cobwebbed dungeon, it seems reasonable that he asks the party for a short rest so that he can memorize new spells (not a rest for him, as he would be spending the hour memorizing).
Assumptions I make about the wording:
- It is designed to limit the Wizard class's flexibility, and thereby its power
- The game mechanic of a long rest is intended to represent the erasure of a wizard's spell specific memory - and thus it is free to receive new spells
Can a wizard partially switch out his spell list? In other words, can he trade rest duration to replace a smaller number of spells during a short rest?
Note: I have asked several questions here that are at the limits of printed rules. I understand that I can modify the rules as I see fit as the DM. The point here is that I am working with several young people, and I don't want them walking into a AL or other 5e game and not understanding the nuances of the game based on learning house rules the first time.