How do I manage contacts who change companies or quit?

How do I manage contacts who change companies or quit?

Hi, 

In the industry I work, we often see people changing employers. Company A and Company B are both our clients. Mr John Smith was a client of us when he was with company A. But since a couple days, he now works at Company B and he is still our main contact in the new company. 

How should I manage him? 

I thought of those options but none satisfies me: 
1- Create a new contact and keep the ''old'' one with ''OLD'' written in his name. 
2- (I actually did that and I'm not sure if thats good) i just modified the person email and the company information.  But I lost the email history. 


What if he simply left and is no longer a client?

Is there a way to flag him as ''inactive'' or ''no longer relevant'' or something else?

Thank you for your time