James Gorman, who takes over as Morgan Stanley's chief executive on January 1, plans to put finance chief Colm Kelleher and banker Paul Taubman in charge of the company's institutional securities unit, according to a source familiar with the matter.

Gorman has been preparing the management changes and others in anticipation of taking the helm at the firm, the source said. An announcement explaining the changes is expected as early as Tuesday.

Morgan Stanley declined to comment.

In September, Morgan Stanley said Gorman, the bank's retail brokerage head, would replace John Mack as chief executive.

Mack, however, will remain chairman of the company.

(Reporting by Steve Eder in New York and Santosh Nadgir in Bangalore; Editing by Lincoln Feast and Maureen Bavdek)