Do you mean that Mr. X was the original promoter of a company (thereby his name appears in the Memorandum of Association)?
If this company is now operating then Mr.X (if he is not a Director or Managing Director) - all he has to do is transfer his shares and get his name removed from the Register of Members (care should be taken that membership does not fall below 7 for public and 2 for private companies)
Remember: In the future (if he was director) acts commited during his tenure with his knowledge or connivance will make him liable even if his name now no longer appears..... and if he is a member then he can be liable as a List A or B Contributory if any amount remains unpaid on shares.
Subscriber's name / names of the persons mentioned as Directors in memorandum / articles cannot be changed / altered for the reason that There is no change in the fact that they are the subscribers / first Directors