What happens when my opponent plays Switcheroo on their 2 [M] unit and my empowered Gangplank, Naval?

Gangplank, Naval will replace his Might being lowered with him being given +3 [M] this turn.

Swap calculates the difference in Might and generates two effects: an increase, and a decrease. When a swap effect is applied to two units’ Mights, it gives those units +[M] and -[M] respective to who has the higher Might.

Under the hood, Gangplank’s replacement effect sees the event of him being given -[M] and replaces it with him being given +3 [M] this turn instead. The event of the other unit being given +[M] is unaffected by Gangplank’s replacement effect. By the time we apply the replacement effect, the difference between the two units has been calculated. In the example with the 2 [M] unit, the final Mights of the two units will be 2 [M] → 6 [M], and 6 [M] → 9 [M].

The way this works is not immediately obvious based on how the swap game action rules are written. This is expected to be clarified in a future rules update.

Note that this logic also applies to Mel, Newly Awakened and Switcheroo.