You (the employer) have to exit the employee first and then join him again. At the same time, if the employee has not been employed anywhere else for the last 2 years, this process is not required and the period of two years will be treated as non contributory period. However, in order to avoid complications which mat arise in future, it is better to exit him first on the actual date on which he was relived two years back and then join again with the same UAN.