HR policies with regards to the end of service gratuity

Hi,

I need advice on HR related matter:

I joined in Nov 2014. That time the HR manual applicable says that the gratuity is applicable post completion of 1 year of service. Since the same is more beneficial as compared to labour law, the same is applicable.

Now, the company changed the HR manual in January 2018, wherein they mentioned that the gratuity is applicable only for employees completing 5 years. The issue is that they applied the new HR policy retrospectively and not prospectively.

I need advice on whether the HR Policy can be applied retrospectively or not. Is it legally binding?

Can they apply the new policy (with respect to gratuity) on employees who joined when the older HR manual is in force?

Should the older gratuity rule be applicable to all the employees who joined before implementing the new HR Manual?

If you need any additional info, would be happy to share. I would require written advice from your end. Please advise on the charges accordingly.