Princess Plant (Adventure Time 2) and discard One-Shots

[Brf]

That is an immediate change, not a lasting effect, so there is no need to keep the card on the table during the combat. Therefore, it is appropriate to discard it immediately rather than play it to the table. I would think the victim would keep the card in front of themselves to remind them they are gender-changed. I know the original Curse! Change Sex has the victim keep the card until after their next combat because there is a -5 combat penalty.