In with the new..out with the old?

How do you write a marketing plan for an established business that has outgrown it's initial business plan?
A good friend of mine who is a Creative Marketing Director brought this question to me last week. After thinking about it and talking to him we decided to bring the question to my blog.

This brand has outgrown there original business plan and need a new plan without its roots and what got them to where they are today. Sounds easy...but when you get to thinking about it...maybe not so easy.

Is it worth writing a new one? Or forging ahead with the old one?