What happens when you end up sharing an enemy (or friend's) space due to a size change?

Question

The rules are clear about "willingly" ending a move in an occupied square:

"Whether a creature is a friend or an enemy, you can’t willingly end your move in its space."

But they aren't as clear when it comes to sudden changes in size (such as those from a Wild Shape into a larger creature, or when under the effects of an Enlarge Person). If such an effect results in the character instantly sharing the same square(s) as other creatures, what happens? Do they get pushed back to an unoccupied square? Or is the character forced to an unoccupied space (assuming one exists)? If no such unoccupied space exists, is everyone suddenly considered 'squeezing'?

Answer 1

Whether a creature is a friend or an enemy, you can’t willingly end your move in its space.

Emphasis mine.

Consider You and another creature (Zorb). If you are forced to share a space with Zorb, it happens - no shunting in any directions unless something specifically tells you to.
So, if you fail a save against polymorph and are turned into a Large creature (and your new size overlaps with Zorb's space), then you are sharing a space with Zorb.
On your next turn (or Zorb's next turn, if it happens first), you must move to no longer be sharing a space (if it's possible).

That's RAW.

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As far as how I would adjucate it: I would make use of the Squeezing rules.

Squeezing into a Smaller Space
A creature can squeeze through a space that is large enough for a creature one size smaller than it. Thus, a Large creature can squeeze through a passage that’s only 5 feet wide. While squeezing through a space, a creature must spend 1 extra foot for every foot it moves there, and it has disadvantage on attack rolls and Dexterity saving throws. Attack rolls against the creature have advantage while it’s in the smaller space

(thanks KorvinStarmast for the rules text)

I would mandate that you Squeeze so that your Large form only occupies a Medium creature's space (and no overlap happens in the first place), and that you cannot stop Squeezing until there is room for your entire form.

But that is not RAW, and is merely my opinion.

Answer 2

The rule about moving through spaces is on page 191 of PHB, and it reads, as you quoted:

You can move through a nonhostile creature's space. In contrast, you can move through a hostile creature's space only if the creature is at least two sizes larger or smaller than you. Remember that another creature's space is difficult terrain for you.

Whether a creature is a friend or an enemy, you can't willingly end your move in its space.

If you leave a hostile creature's reach during your move, you provoke an opportunity attack, as explained later in the chapter.

Looking further, a few spells have different "occupy the same space rules":

Etherealness

If you occupy the same spot as a solid object or creature when this happens, you are immediately shunted to the nearest unoccupied space that you can occupy and take force damage equal to twice the number of feet you are moved.

Forcegace

Creatures only partially within the area, or those too large to fit Inside the area, are pushed away from the center of the area until they are completely outside the area.

From these, a DM ruling the player is shunted, with or without damage, both seem reasonable rulings and are most likely the RAW or RAI.