A lot of people, when they start with TDD, get annoyed when you say that you may have to rewrite a lot of stuff, including whatever your already wrote.

TDD was designed to throw code away: The more you learn about your problem, the more you understand that, whatever you wrote, won't solve the problem in the long run. Also, as you slowly solve new problems, you may notice some pattern in the code emerging (you're doing the same thing over and over, with only minor changes). That's a good time to go over and rewrite everything to take advantage of this pattern.

You shouldn't worry about this. Your code is not a wall (or any physical object): if you have to throw it away, you didn't wasted materials. Surely it means your time writing code was lost, but you got a better understanding about the problem now, or you may start to think in a more concise way to solve the problem.

Not only that, but as you progress through your project, solving problems and getting "acquainted" with the problem, you'll also notice that the spec will also change. This means that the problem your code solve wasn't exactly the problem you needed to solve; your code is trying to solve something that isn't exactly the problem.

Also, specs changing is really common. One thing that you can be sure is that it won't change everywhere. Some of the things you solved will stay the same, some others will be completely removed and some others added. And you will see that you'll refactor your code a lot, and throw a lot of code away. And not just code that solves the problem, but also the tests for that code.

... unless you focus mostly on integration tests.