If a customer service expert wants to change his career, which would be the most suitable position for him?

I think it will be the sales field ..
The both are very familiar.

It's all up to her or his aspiration and interest, not in any stereotypias linked to professions.
Architecht, physiotherapist or writer maybe? Be true to yourself, the answer is there.

the best position is salesman

Customer relation..or Sales..hospitality are really linked

A Nurse ....

Relationship Manager, Marketer, Process review Manager, HR assistant

that would be Key ACcount Management, Business Management, Business Analysis in terms of Service Delivery Aspect.
Syed Waqas Saghir Internet Marketing Expert, SEO Consultant, Social Media Evangelist.

1.
Make a list of what you have liked and disliked about your past jobs.
Consider what do like to do in your spare time.
What are you passionate about? If you could pursue your dream job, what would it be? 2.Make a list of the skills you have obtained through life.
This list can include both soft skills, like organizing, time management, writing and communicating, and hard skills such as software platforms you have learned.
Once you have created this list, think about how you might leverage these skills into a new career.
3

I would suggest him/her to join some kinda philanthropy services, relationship management, any Non-Government or Non-Profit Organization.
A customer services professional can also become a good public speaker, but for this he/she has to specialize in any specific topic e.g.
moral development, personal grooming or effective speaking etc.

marketing and Management of the institutions are better