What happens if switcheroo is played on forbidding waste where one player the attacker has a 5 might unit and the other has a 1 might unit defending

They swap — the attacker's unit becomes 1 Might and the defending unit becomes 5 Might, and Forbidding Waste's -2 keeps applying to the defender on top of that.

Switcheroo swaps the units' current Might (CR 434.1.b: take the difference, increase the lower value and decrease the higher), so the battlefield's -2 is already baked into the defender's 1 when the swap is calculated. The -2 then continues to apply afterward, because Forbidding Waste is a passive "while defending alone" effect that re-evaluates continuously — the defender is still defending alone, so nothing falls off (FAQ #10921).

Practical consequence: since damage uses current Might at the Combat Damage Step (CR 467.2), the 1-Might attacker assigns only 1 damage to the 5-Might defender, while the defender assigns 5 to the attacker's unit — almost certainly killing it. Note the swap only lasts this turn.