Tests written while doing TDD absolutely are valuable throughout the lifetime of an application.
The purpose of TDD is to allow you to build up your code test by test, so that it always meets the requirements you've implemented thus far (this is the "works" part), is fully tested and is well factored. When you're done you'll have a full regression test suite as a bonus, which is valuable. So for both proving what requirements you've implemented and for regression, it's valuable to keep your tests running.
If requirements change so fast that you can't maintain the test suite, you have a project management problem. Explain to your customers that you don't have enough time to ensure quality, or that they need to hire a test engineer.