Can you change to a fly speed if you suddenly find yourself in mid-air?

Question

I am aware of the following related question, which establishes that you can change to a fly speed at least when you teleport yourself into the air, but there are other times where I am unsure what would happen:

• Can I Misty Step into Midair?

The section on "Using Different Speeds" states:

If you have more than one speed, such as your walking speed and a flying speed, you can switch back and forth between your speeds during your move [...]

What if you all-of-a-sudden want to use a different speed; how does changing from one speed to another work?

Imagine the following scenario: A creature, who has a fly speed, but is currently standing on the ground, gets launched into the air, or has the ground falls out from under them.

• Do they fall?
• Can they begin flying?
• Does it matter whether or not it is currently their turn?

Is this a circumstance where a creature can change their speed, similar to teleporting yourself, or is this somehow different because it may not be during their move?

I would like answers to either be able to account for the fact that you can start flying after teleporting, or be able to refute that claim.

Answer 1

Speed is irrelevant if you aren't moving

So what if you suddenly want to use a different speed; how exactly does this shift from one speed to another work?

Whenever you switch, subtract the distance you've already moved from the new speed. The result determines how much farther you can move. If the result is 0 or less, you can't use the new speed during the current move.

You say "I walk 10 feet, now I fly 20 feet, now I swim 5 feet" - providing you have at least 10 feet of (normal) speed, 30 (10 + 20) feet of fly speed and 35 (10 + 20 + 5) feet of swimming speed OR 40 (10 + 20 + 5 x 2 for difficult terrain) feet or (normal) speed, this all works.

For example, what happens if a creature, who has a fly speed, but is currently standing on the ground, gets launched into the air?

They start to fly. From Flying Movement:

If a flying creature is knocked prone, has its speed reduced to 0, or is otherwise deprived of the ability to move, the creature falls, unless it has the ability to hover or it is being held aloft by magic, such as by the fly spell.

Unless whatever launched them into the air, knocked them prone or reduced their speed to 0 or rendered them unable to move they just start to fly - they can't move until their turn of course. If it did do any of those things, they fall (unless their fly speed comes from magic).

Is this a circumstance where a creature can change their speed, similar to teleporting yourself, or is this somehow different because it may not be during their move?

They aren't moving - speed is irrelevant. Either they can fly or they can't fly. If they can, they fly; if they can't they fall.

Answer 2

Outside of your turn you can switch from walking to flying.

Question 1:

So what if you suddenly want to use a different speed; how exactly does this shift from one speed to another work?

Answer:

If you have more than one speed, such as your walking speed and a flying speed, you can switch back and forth between your speeds during your move. Whenever you switch, subtract the distance you've already moved from the new speed. The result determines how much farther you can move. If the result is 0 or less, you can't use the new speed during the current move.

Question 2:

Imagine the following scenario: A creature, who has a fly speed, but is currently standing on the ground, gets launched into the air, or has the ground falls out from under them.

This is actually a really good question. Can you switch to a state of flying outside of your turn? It is wrong to assume that not being able to switch to a new speed means you can't change from walking to a state of flying or swimming. What you quoted is under the subheading "using different speeds" found under a larger subheading "breaking up your move," therefore it is specifically referring to your movement within your own turn "If the result is 0 or less, you can't use the new speed during the current move." It has nothing to do with what your character can do outside their turn. Frankly, it doesn't even say you can't switch from walking to flying, it says you cannot switch to the new speed; put plainly, you cannot switch from walking speed to flying speed. Speed is a rate of displacement, flying speed is the rate at which you can displace your own position using flight. You then cannot displace yourself using the new mode of movement, because you cannot use that speed.

Then, is there anything that refers to your movement out of turn? Under movement and position it does say:

In combat, characters and monsters are in constant motion, often using movement and position to gain the upper hand.

On your turn, you can move a distance up to your speed. You can use as much or as little of your speed as you like on your turn, following the rules here.

The first part of the quote refers to how creatures are moving in combat, they are in constant motion, they don't stall in their movement to allow the enemy a turn. They can change how they are moving however they want to gain the upper hand. The second part cements what I said earlier, your speed is used in reference to movement, or displacement you can achieve on your turn.

You can 'move' however you want outside your turn, you just can't achieve a displacement of position which is what your speed is.