The moves were made before the start of practice Wednesday night.
Westerman has four years of NFL experience after splitting last season between Arizona and Indianapolis. He spent his first three seasons with the New York Jets, where he played under current Bills defensive co-ordinator Mike Pettine.
Overall, Westerman has 53 tackles and 5-1/2 sacks in 45 career games.
Caussin was designated as waived/injured. He's not practiced since hurting his hip on Aug. 2. The second-year player has had little luck since appearing in six games with Buffalo in 2011. He missed all of last season because of a knee injury.
The Bills (1-0) host Minnesota (0-1) in a preseason game Friday.