[jakejake]

I'd consider it a positive thing if a client asks if they can change specifications mid-project. I could explain to them how the price will also need to be flexible and possibly talk them into doing the project using an agile methodology.

That's one thing that's difficult with fixed bid projects is that you often have to resort to BDUF with client sign-off on everything in order to protect your own butt. Even though agile may be much better for some projects. Clients are understandably squirrelly about open-ended prices.