Growth is disruptive. It requires change - all the time. There is nowhere that this is more true, and more difficult to deal with, than in the area of your client/customer base. It's unfortunate, but the customers that get your business to one level are often the barrier to getting you to the next level of growth and profitability.
The only way a fast growth company can sustain fast growth is to find ways to increase its leverage. Fast growth must be geometric, not arithmetic. As your business grows, so does it complexity and its cost structure. You must find customers who are capable of paying you a multiple of that increase. The failure to do that will cause the business to be overwhelmed by complexity, and it will stall and stop growing.
Additionally, the most important asset you have to support fast growth is focus. You must serve a core customer base better than anyone in the world. If you growth plan has you catering to larger customers, or customers dealing with bigger problems, continuing to focus on your legacy customers merely holds you back from fully serving your future ones. Its an unfortunate truth about growth.
Sustainable fast growth is all about allocating resources to your highest value activities. And sometimes, that means you have to say goodbye to people that were very important to you.