Dear Rajesh,
I infer two things from your post - may be from the point of view of your boss.
One is testing your HR knowledge related to hiring and firing of people in the interest of the management free of any legal hassle. If not, the second one is to ascertain your capability to implement the change desired by the management in staffing pattern. Therefore, you have to handle the situation with utmost caution.
Once employed on regular basis in an organization whether it is industrial or otherwise, every employee earns a statutory right to security of tenure of his employment. It can't be simply taken away by the employer as and when he changes his hiring policy. If the employer wants to switch over to indirect methods of employment like contract labor system, FTC employment alternatively in respect of any category of existing job positions occupied by regular employees, such a move, apart from being unethical, would lead to industrial unrest or Disputes. As a HR manager, how you handle it by protecting both the interests of the management as well as the employees likely to be affected would be an acid test to your crisis management skills. You can not work out the economic gains of this change over as expected by your management in isolation : rather the cost of terminating the regular security personnel in a sudden and single go has also to be considered without fail. The psychological impact of such a move likely to create on the morale of other regular employees in the organization has also to be simultaneously considered.
Therefore, as a first step on your part , ensure that you have politely brought out all these procedural bottle necks to the notice of your boss before implementation. Second, either formally with the prior permission of the management or informally, you may initiate a dialougue with the concerned employees and assess the possibility of a voluntary separation with additional terminal benefits.
Of course, the final decision is with the management only. But there should be no complaints later that you failed to bring the consequences to the notice of the management or handled the situation so haphazardly to the disadvantage of or disrepute to the organization.