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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:
...or something 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.
As soon as you start casting a spell or using a special ability that requires concentration, your concentration on another effect ends instantly. (XGE p. 5 - Introduction: Ten Rules to Remember)1
Levitate takes an action to cast which is some non-zero amount of time. So, in between the time you start casting and when you complete casting, you start falling.
The default rule in the PHB (p. 183) doesn't specify exactly how fast a creature falls.
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.2 This means that falling precludes the casting or completing of spells since both of those activities happen slower than "immediately".
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.
1 - For one type of ambiguity that this rule was likely created to clear up see Can a concentration spell be cast without actually concentrating on it for an "instant" effect?
2 - Note that if you are falling from above 500 feet and are using the optional rule for falling in XGE this would not be true. In that optional rule you fall up to 500 feet per turn (depending on if you hit the ground or manage to arrest the fall before then). In that circumstance and with that rule you would have time to take another turn while falling.
So, the below was my thinking from the Player's Handbook rules. But those are out of date. In fact, your spell is lost as soon as you begin to cast another that needs concentration; this is based on Xanathar's which contains clarifications, or in this case complete reversals, of certain rules. So, assuming you're not using any alternate rules than the core ones, you immediately fall the full distance - and are probably dead.
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 spell is still up and you cast levitate, then you are under the effect of levitate when your fly ends, because it ends when you cast another spell. It must be in effect, because if it's not, then you have not cast it.
The wording on the rule about concentration 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 levitate, and do not finish because I fall to my death, I did not cast levitation. I began casting levitate, but I did not cast it. So you do not fall to your death before levitate takes effect.
As for the impact of the levitate spell, 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 or implies in any way that you must be on the ground to be effected, so there is no reason you would drop.
Xanathar’s Guide to Everything has two rules about falling on page 77, the first is the Rate of Falling:
The rule for falling assumes that a creature immediately drops the entire distance when it falls. But what if a creature is at a high altitude when it falls?
[...]
When you fall from a great height, you instantly descend up to 500 feet. If you're still falling on your next turn, you descend up to 500 feet at the end of that turn. This process continues until the fall ends, either because you hit the ground or the fall is otherwise halted.
Using this optional rule, you could rule that your character starts to fall but, as they have used levitate to halt the fall, you stop falling and will not be falling when you start your next turn. Theoretically, you could fall anywhere from 1 to 500 feet as the rule in XGE states you fall “up to 500 feet” not that “you fall 500 feet or hit the ground, whichever comes first”. I would imagine that the DM would determine how far a character falls each turn, possibly falling a further distance each turn to simulate gaining speed.
There is also a rule specifically about Flying Creatures and Falling:
A flying creature in flight falls if it is knocked prone, if its speed is reduced to 0 feet, or if it otherwise loses the ability to move, unless it can hover or it is being held aloft by magic, such as the fly spell.
[...]
Subtract the creature's current flying speed from the distance it fell before calculating falling damage. This rule is helpful to a flier that is knocked prone but is still conscious and has a current flying speed that is greater than 0 feet. The rule is designed to simulate the creature flapping its wings furiously or taking similar measures to slow the velocity of its fall.
According to this section, the “flying creature” (the character) would begin to fall as soon as they were no longer “being held aloft by magic, such as the fly spell”. As they no longer would have a flying speed, due to the spell being cancelled, the last section would not normally apply.
However, as the character casts levitate as they were falling, this would come under “taking similar measures to slow the velocity of its fall“. You would subtract 20 feet from the fall as levitate’s description says you “rise vertically, up to 20 feet”. Theoretically, if you were only 20 feet up in the air when you cancelled fly and cast levitate, you would take no damage.
The last part of the rule for Flying Creatures and Falling focuses on if you combine this rule and the first one for Rate of Falling:
If you use the rule for rate of falling in the previous section, a flying creature descends up to 500 feet on the turn when it falls, just as if that creature starts any of its later turns still falling and is prone, it can halt the fall on its turn by spending half its flying speed to counter the prone condition (as if it were standing up in midair).
What this means is, when you cancel fly, you will start to fall. However, when you cast levitate, you can use 10 feet of movement in order to stop yourself from falling.
So, whilst you would not be able to flawlessly switch to using levitate, it is possible to do so mid air without hitting the ground and taking damage. However, you would always fall at least some distance (even if the DM rules it is only a few feet) so the character would not be at the same height.
