If I switch to the Tarifa Constante, do I lose the Price Guardian?

Yes. The Price Guardian is only available to Tarifa Transparente customers. If you switch to the Tarifa Constante, the Price Guardian is automatically deactivated and you become ineligible to activate it for as long as you remain on this tariff. If you switch back to the Tarifa Transparente in the future, you'll be able to activate it again.