Suppose a character has cast fly on themselves, and wishes to switch to levitate to prevent themselves from taking fall damage if they lose concentration.

What happens if they cast levitate while in midair? Do they

1. Stay at the same height


2. Fall immediately to 20 feet


3. Fall immediately to the ground

...or something else?

3. RAW they fall instantly with no chance to cast anything else

You start falling as soon as fly ends, which is the same instant you start casting levitate. Starting to cast one concentration spell instantly ends any other concentration spell you have going. Levitate takes an action to cast which is some non-zero amount of time. Falling however, is instant.

PHB rules are unclear: entirely up to your DM

The rules on falling in the PHB/basic rules state:

A fall from a great height is one of the most common hazards facing an adventurer. At the end of a fall, a creature takes 1d6 bludgeoning damage for every 10 feet it fell, to a maximum of 20d6. The creature lands prone, unless it avoids taking damage from the fall. (PHB, p. 183)

If you go strictly by PHB rules only, then there is no answer and the DM must decide.

Falling is instant according to XGE clarifications.

However, using the clarification from the preface to the optional rules on falling in Xanathar's Guide to Everything makes this default rule much clearer.

The [PHB] rule for falling assumes that a creature immediately drops the entire distance when it falls. (XGE, p. 77)

So, using the default rules as clarified by XGE, a creature has no opportunity to do anything once they start falling. They immediately fall the entire distance and take however much damage or other effects they have triggered.

So as soon as you stop being affected by fly, you will fall the entire distance before you have a chance to cast another spell or do anything else.

Note that fly will end as soon as you start casting levitate because concentration ends when you start casting another concentration spell.

I'd rather be commenting this on the above answer, but I don't seem to be allowed to.

From my reading of the rules on concentration, concentration on a spell ends when you cast another one that needs it. So if your fly is still up and you cast levitation, then you are under the effect of levitation when your fly ends, because it ends when you cast another spell. It must be in effect because if its not then you have not cast it. The wording is "You lose concentration on a spell if you cast another spell that requires concentration" not you lose concentration on a spell when you begin to cast another that requires concentration. If I start casting levitation, and do not finish because I fall to my death, I did not cast levitation. I began casting levitation, but I did not cast it.

As for the impact of levitation, it states that the target "rises vertically, up to 20 feet". So you move up to 20 feet higher than you were when you were flying. It never suggests there is a maximum height off the ground you can be.