Let me be clear, is't like joined in an unregistered co. and later on this was regd. and continued it's business/with same employees as in the past ?
Change in status of the employer does not impact in change in the joining time of its employees. Ofcourse in instances of employees continued but change in the names of entity (unregistered to regd.) it might. This can be overcome by issuing a formal letter to everyone stating the continuity of service is validated without any break. This formality would ensure full entitlement in respect of EPF/EPS, Gratuity, bonus, confirmation, increments & promotions. However it's inevitable/essential to appropriately arrange name change, if any, in statutory documentation in registrations such as EPFO, ESI, PT, PAN/TAN/TIN, GST/ Labour licences, MSME etc.
22nd December 2018 From India, Bangalore