Sullivan & Cromwell on Wednesday announced a changing of the guard, with plans for H. Rodgin Cohen to assume a senior role at the firm and partner Joseph C. Shenker to take over as chairman of the prominent Wall Street law firm on Jan. 1.

Shenker, 53, had been expected to assume the role of chairman since February 2006, when the firm named him vice chairman as part of a succession plan that would have Cohen stepping down by age 65. Cohen, who has been chairman since 2000, will remain on the management committee and assume the title of senior chairman, though he is expected to primarily focus on his practice. “Joe’s proven to be an outstanding leader,” Cohen said in a firmwide e-mail. “As the firm’s senior chairman, I look forward to spending even more time with the firm’s clients.”