Technically speaking YES.
Entity is the same.
Proprietorship changed ( status modification ) to Pvt.Ltd.,
Ltd., by shares.
Who is the director ? What does the articles of association and memorandum of association say ?
What is the paid up capital of the new entity ? for that paid-up capital, what is the source of funds ?
What is the PAN of the company ? What was the PAN of the proprietor ? What is the new PAN of the director who was earlier the proprietor ?
So legally if some employee goes to court, and pulls out all records, one after the other, and establishes that his/her contribution as an employee
has been to the same BUSINESS ENTITY ( but with status modification from proprietor to ltd., firm ) then YES, the employee has a huge chance of winning the matter in court.
But, strictly speaking, as per the payment of gratuity act, you may say NO since the five year term comes in-between as an obstacle.
To avoid future complications, if they write to you seeking gratuity or even voluntarily you may make the pay-out.
In fact it helps in the future to even prove your credentials as one of an ethical businessman.
For more practically legal tips, I suggest, you speak to any retired ( recently retired ) labour commissioner and then meet a few advocates.
But first meet a retired labour commissioner since it is better that you get practically legal tips.