As of right now, it's not clear who has been released to make room for Florence. One would assume a defensive back like Jonte Green or Ricardo Silva would be released and signed to the practice squad to make room for Florence, but nothing has been officially announced just yet.
While Florence has certainly seen better days, he does serve as an upgrade for the Lions' secondary. He brings a wealth of experience, and while he's had a tough time sticking with teams as of late, right now he's a good fit for the Lions' roster. The hope is that the Lions will eventually get to a point where they won't need to make moves like this right before the season, but the secondary just isn't there yet.
After signing with the Lions, Florence sent out this tweet on Sunday:
A setback is nothing but a setup for a comeback < Detroit Lions > let's get ready for St. Louis— Agent 29 (@DraytonFlorence) September 2, 2012
In 2011 with the Bills, Florence started all 16 games and had 50 tackles, 12 pass defenses, three interceptions, one touchdown and one forced fumble. I think the Lions would be very happy with numbers like that in 2012.