If you have 500 customers and your competitor has 500 customers. You are in a market of over 10,000 potential users. What do you do? go after your competitor's

If you have 500 customers and your competitor has 500 customers. You are in a market of over 10,000 potential users. What do you do? go after your competitor's 500 customers, many of which are only going to consider switching if their product lets them down or go after the other 10,000 potential customers that don't use either product?