Hi Jyoti,
Come on, you are saying "having the same responsibilities but at a different level." When there is a change in level or title, think of mentioning it as a "promotion" to bring some happiness to the employee. Anyhow, I am giving you the format:
Dear Mr. [Last Name],
We are pleased to re-designate you in the grade of [Grade] with effect from [Effective Date].
However, there will be no change in the terms of your appointment or roles and responsibilities. We expect your continued contribution in achieving excellence.
Yours truly,