(Reuters) - Blackstone Group LP (>> The Blackstone Group L.P.), the world's largest alternative asset manager, said on Thursday it had extended the vesting period for shares given to employees, a move that could help retain investment bankers in a unit to be spun out this year.

Blackstone announced last year it would combine its mergers and acquisitions, restructuring and private fundraising advisory businesses with the advisory boutique of star investment banker Paul Taubman. The combined entity is expected to be spun out to Blackstone shareholders in the second half of 2015.

Blackstone employees receive on average 15 to 20 percent of their compensation in shares, and are entitled to these shares in four annual installments. They were previously allowed to keep these shares even if they left the New York-based firm in the interim.

The change in the vesting schedule, which was made in the fourth quarter, means that Blackstone employees in its advisory businesses could lose shares they were entitled to as compensation if they left early, making them less vulnerable to recruitment pitches from other investment banking boutiques such as Moelis & Co (>> Moelis & Co) and Evercore Partners Inc (>> Evercore Partners Inc.).

"The shares immediately vest, we changed it to three years. This has retention characteristics that are helpful, so that impacts advisory and (Blackstone's hedge funds investment arm) BAAM in particular," Chief Financial Officer Laurence Tosi told investors and analysts on Blackstone's quarterly earnings call.

The change has less impact on Blackstone employees who work on private equity, real estate, and other such funds. These funds already pay employees who invest in them performance fees, also known as carried interest, under a three-year vesting schedule.

Blackstone pays its employees carried interest in cash. On the other hand, Apollo Global Management LLC (>> Apollo Global Management LLC), a rival alternative asset manager, does use shares to pay carried interest.

(Reporting by Greg Roumeliotis in New York; editing by Gunna Dickson)

By Greg Roumeliotis