A tentacle with 1 might is defending on Forbidding Waste and a sprite with 3 might is attacking. If a Switcheroo is used, what are the mights of the units after?

Tentacle ends at 3 Might; the Sprite ends at -1 (treated as 0 for all game purposes).

Forbidding Waste's -2 applies because the Tentacle is defending alone, so its current Might is actually -1 (not 0) when Switcheroo calculates the swap — negative values are used in the math (FAQ #4763). Switcheroo reverses -1 and 3, giving the Tentacle a +4 increase and the Sprite a -4 decrease.

Layered arithmetic: Tentacle = 1 base + 4 (swap) − 2 (Forbidding Waste) = 3. Sprite = 3 − 4 = -1, which Rule 143.2.b treats as 0 whenever referenced.