From his Senate page:
"Elected to the U.S. Senate on November 5, 1996 and re-elected in 2002, Durbin fills the seat left vacant by the retirement of his long-time friend and mentor, U.S. Senator Paul Simon."
So he switched his position while still a US Rep, before he became a Senator.
My guess is that he wanted to run for the Senate, and he had to change his position in order to get the nod by the democrat senatorial committee.
check this link at this post out
http://www.freerepublic.com/focus/f-news/1555919/posts?page=461#461
He wanted an amendment to over turn Roe
Hey Senator Corbin .. Thank you for mention Durbin's pro-choice record!!