another question, can I respond to a Volrath's Shapeshifter becoming a creature? Say my opponent discards a creature into a grave with Shapeshifter in play. Can I in response to Shapeshifter's becoming a copy ability bolt the Shapeshifter?
It depends on how the card is discarded. With Shifter's ability it's an effect and can be responded to, but with say Survival of the Fittest or Wild Mongrel, it's a cost, and can't be responded to.