Offer letter reissue joining date - company converted from proprietary firm to public limited company

I work in IT company as an HR. The company was proprietary firm and from 2017, it has got converted to the public limited company. We did issue offer letter to all employees for proprietary company but since now the company is public limited, do i have to issue new offer letter on public limited company letterhead, if so then what joining date and salary I should write in offer letter for 5 years experienced employee?

The name of entity that had issued letter does not legally exist. As such you have to issue two lined letter to all the employees: line one, informing them of company name changed from X to Y; line two, informing them that the terms & conditions of earlier letter dated --- will remain unchanged.
Needless to say that the letter should be printer on new company letter head & employees should acknowledge copy of the same.