Write a letter informing about the change in firm's status.

To Whomsoever It May Concern:

This is to bring to your kind attention to the fact that North-Eastern Distributors are now registered as Private Limited Company with effect from Date_________ 

You are, therefore, kindly requested to mail all your correspondence, orders, bills and cheques/drafts in favor of North-Eastern Distributors (P) Ltd. to the above mentioned address. 

Thanking you for your kind cooperation.  

Sincerely,