In your question it is not clear whether the number of employees at the time of being proprietory firm was less than ten. The Payment of Gratuity Act shall apply to a factory / mine/shop/establishment etc. where ten or more persons are / were employed. If at the time of being propritory firm the company was having total number of empoloyees less than ten and after conversion to Privet ltd company the total strength becomes ten or more than ten, Gratuity Act be applicable from then only and not before.
Rgds,
Debrishi
Rgds,
Debrishi