hairstyle switch
When two characters sport each other's usual hairstyle. They don't necessarily have to be switching their hair length as well.
If only one character is present, use borrowed hairstyle instead.
If the character only switched their hair length but not their hairstyle, use hair length switch instead. If the characters switched both their hairstyle and hair length, use this tag and hair length switch together.
See also:
This tag implicates alternate_hairstyle (learn more).