Does Not so Fast counter Switcharoo?

Yes, Not So Fast can counter Switcheroo.

According to RiftJudge FAQ #8679, this interaction works because Switcheroo explicitly chooses (targets) two units when it is played. Because it targets units, it meets the requirement for Not So Fast, which states it can "Counter an enemy spell or ability that chooses a friendly unit or gear."

How the interaction works:

1. Targeting: When Switcheroo is played, the player must select two specific units as targets.
2. Reaction Timing: You must play Not So Fast while Switcheroo is still on the chain (before it resolves).
3. Resolution: If Not So Fast is played, Switcheroo is countered. Per Rule 425.1.a, the countered spell does nothing and is cleared from the chain. Per Rule 425.1.c, the energy spent to play the countered spell is not refunded.