Because damage remains marked on a creature until the cleanup step or an effect removes that damage, nonlethal damage dealt to Dragon's Rage Channeler may become lethal if the number of card types found in your graveyard drops below four.

The card types that can appear in a graveyard are artifact, battle, creature, enchantment, instant, kindred, land, planeswalker, and sorcery. Legendary, basic, and snow are supertypes, not card types; Human and Shaman are subtypes, not card types.