YARD to POOP.

Every letter change

1. YARD → BARD: change letter 1 from Y to B.
2. BARD → BARN: change letter 4 from D to N.
3. BARN → BORN: change letter 2 from A to O.
4. BORN → BOON: change letter 3 from R to O.
5. BOON → BOOP: change letter 4 from N to P.
6. BOOP → POOP: change letter 1 from B to P.