If he has been allotted Social Security Number by the Employees Provident Fund Organisation, that number can be retained. For centralised pay system wherein an employee's pay is released at Hyderabad and contribution is deducted and deposited in Hyderabad, but is transferred to Delhi office of the same organisation, then also the number will be retained. However, if these two establishments have different PF number, then the number will change.
The EPF account number has prefixes as Regional Code (in which region the establishment works) and an establishment code (where an employee works) which will be common to all employees under such an establishment. On assuming duty in Delhi, he will be allotted a new number. However, his past PF accumulations can be trensferred to the new account by sending form 13 duly signed by the present employer.
Regards,
Madhu.T.K