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.
Best Answer
This is one of those changes from 3.5e to 5e D&D
There was a mechanic in D&D 3.5e wherein a Wizard or other spellcaster that uses the spell preparation mechanic could in essence leave a few of their spell slots unprepared so that later they could be filled as necessary with spells that they might need throughout the day. These "late preparations" took a minimum of 15 minutes but allowed the spellcaster to get a deal of utility out of spells that had a non-combat role. The caveat was that the slots had to be empty and you couldn't drop a slot you had prepared a spell in to prepare a new spell unless you had just finished a full rest period.
The new edition introduced ritual spells to cover some of the need for this
Preparation in 5th edition changed a great deal from prior editions of Dungeons and Dragons. The 15 minute spell prep by leaving spell slots empty isn't really required anymore due to the ritual mechanic which allows using 10 minutes to cast non-combat spells (that have the ritual tag) outside of combat. This provides the Wizard with a lot more versatility on how to choose the spells they'll use to deal with the challenges they believe they'll face that day. (Gonna be in the middle of the desert? Probably won't need feather fall.)
The 5e rules also opened up your spell list so that you can prepare less of one spell level and more of another, giving many choices on how many spells of each level you'd like to prepare. You are no longer locked into 4 first level spells and 2 second level spells. You could choose to prepare 2 first level spells and 4 second level spells so you have more of the useful spells available to account for your daily challenges and to get the most out of your higher level spell castings.
What is your DM's preference?
Fifth edition never states that you can't prepare five of your six daily spells and leave the six slot open to prepare it later as you may need it. It also doesn't explicitly say that you can do that either. It's a decision that will most certainly have to be made by your GM. For instances where the book doesn't explicitly state what you can and can't do, the GM is responsible for making a call that might or might not allow you to do that thing.
Is it needed?
The usefulness of this is actually a great deal less than in 3.5. Due to the availability of ritual spells and the greater flexibility in spell preparation, there is less of a need for leaving empty slots in 5e than there ever has been in D&D. In short, the introduction of ritual spellcasting has removed the need for an empty slot / late preparation requirement that just wasn't present in prior editions of Dungeons and Dragons.
Each caster class is treated a bit differently.
Check the PHB for specifics for each class to understand how the ritual tag applies.
The Cleric doesn't get the same benefit, for example.