Set Kingpin move in lane with 2 buffed units and opponent Watchful Sentry is in the battlefield. My opponent plays Switcharoo to Sett. What happens?

When your opponent plays Switcheroo on Sett, Kingpin and Watchful Sentry, the following interaction occurs:

1. Calculation of Current Might

Before Switcheroo resolves, we determine the current Might of both units:

• Sett, Kingpin: Has a base Might of 5. Including his passive bonus, his total Might is calculated based on the number of buffed units present.
• Watchful Sentry: Has a printed base Might of 1.

2. Switcheroo Resolution

When Switcheroo resolves, it calculates the difference between their current Might values and applies a persistent modifier to equalize them according to the swap.

• The Modification: Switcheroo applies a persistent modifier to both units to equate their Might values based on the difference at the time of resolution.
• Result: Sett, Kingpin and Watchful Sentry will have equalized Might values for the remainder of the turn.

3. Key Nuances

• Modifier Persistence: Switcheroo applies a persistent modifier that lasts for the turn. This is not a snapshot of base values; it is a calculation based on current Might. If Sett's passive might bonus changes later this turn (e.g., if one of the other buffed units leaves the battlefield), his Might will be recalculated: (New Base + New Passive) + Modifier. Therefore, his might is not "fixed" for the rest of the turn; it remains subject to further passive or buff changes, which are calculated on top of the modifier (FAQ #9335, #9201).
• Buffs and Passives: Buff counters (physical objects on units) and passive abilities (like Sett's bonus) remain active and attached to their original units after the swap. They are not transferred. Switcheroo only adjusts the final Might value by applying the persistent modifier (FAQ #2144, #341).

Summary: After Switcheroo resolves, Sett, Kingpin and Watchful Sentry will have equalized Might for the remainder of the turn, subject to any further changes in Sett’s passive or buff status.

Note: I could not fully verify this ruling against the retrieved rules.