Is it necessary to change underwear if it has been touched by madhy, wady, or mani?

Semen is pure according to the preponderant opinion. As for wady (thick white discharge) and madhy (pre-ejaculatory fluid), they are impure according to most scholars. If underwear is stained by madhy or wady, the impure spot must be washed before one intends to pray in it, or the garment must be changed.