Dear Bapu,
I think that the provision for fixed term employment finding place in the recently passed Code on Industrial Relations,2020 has prompted your question.
Permanent employment, basically being a contract between the employer and employee, would normally last till the employee's attainment of the pre-fixed age of superannuation. Before that, it can be unilaterally terminated by the employee concerned on his own will by means of resignation subject to the notice clause of exit mentioned in the contract. Similarly, it can also be unilaterally terminated by the employer by means of discharge simpliciter as mutually agreed already in the contract or by means of dismissal on the ground of proven misconduct, if any after duly following the principles of natural justice. Therefore, just because Fixed Term Contract Employment has legal sanction, no employer, on his own volition, can change the permanent position of an employee to that of fixed term contract.
Of course, after retirement either on attaining the age of superannuation or on voluntary basis, the employer can offer the same position on fixed term contract basis to the same person. Here too, if his resignation is under a Voluntary Retirement Scheme, exemption of income tax would be available subject to the provisions of sec.10(10C) and the prohibition of the re-employment of the individual in the same organization or in its subsidiaries, if any.