The exchanging of two characters's usual traits or roles. For example: Character A has Character B's height and vice versa, or Character A and Character B doing each other's supposed usual chores.
Although similar, these two tags should not be confused with "role reversal":
costume switch: two characters wearing each other's default clothes and nothing else
personality switch: two characters having each other's known personalities and nothing else
reversible couple: two characters acting dominant and submissive to each other
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