you can inform them one by one by means of letters with company seals, emails, text messages, telephone calls, etc.
general content: to dear customers: Chen Moumou, who was in charge of XX business in our company, officially terminated the labor relationship with our company on X, X, X, and from now on, the XX business that Chen Moumou was in charge of will be changed to Li Moumou (contact information: ……), and all the comments made by Chen Moumou in the future will no longer represent our company. Please pay attention. Please forgive me for any inconvenience. XX company x year x month x day
if it is a core customer, it is best to let the new business personnel personally call to tell the situation, and also have a preliminary understanding and exchange with the customer by the way.