If I have Volcanion EX in play and I use its "Steam Up" Ability, can I then use Ninja Boy to switch it with a different Volcanion EX and use "Steam Up" from the new Volcanion EX?

No, that won't work. Even though the new Volcanion EX is a different Pokemon than the first one, it remembers the Steam Up ability was already used this turn so you cannot use it again on that Pokemon.

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