There is nothing linear about starting and building a business. There are plenty of twists and turns along the way and successful companies are not only are open to change from their original ideas but build systematic processes for checking and rechecking their business assumptions during the early days of their company.
The idea really hit me this summer at TechStars, when I began thinking about the idea of company creation as "agile" (akin to the agile development methodology) watching how fluid the most successful TechStars companies were. They were constantly checking and rechecking the assumptions behind their businesses and reacting to what they were learning from prospective customers, early users and mentors. These check-ins were very deliberate and occurred at regular intervals.
To me it’s the perfect model (and "agile" is the perfect analogy) for the early days of a start-up. Be flexible. Seek outside input. Be introspective. Stop and consider what you’re learning and if it effects key assumptions behind your business idea. Tweak what you’re doing. Repeat.