What letter can you replace in "luck" to make it an entirely new word!?

From Jubal Phone Pranks from The Jubal Show The Jubal Show

A Jubal Phone Prank is when our listeners set-up a friend or family member to wake up with The Jubal Show, phone prank style. What letter can you replace in "luck" to make it an entirely new word!? In today's Phone Prank Pete Eakins aka Jubal Fresh...