Moving forward, it should be easy to implement any change necessary. This might include a new feature, a change in requirements, or a discovered defect. That isn't to say that the application is complete or error-free, but you should have some level of confidence that the application behaves in the ways accounted for with the existing test suite.
For the purpose of clarification, we are defining optimization as anything that obfuscates the code, making it less clear or more difficult to understand, or anything that limits the possibilities further than the test requires. A premature optimization is an optimization that is done for any reason other than specified by a requirement.
Typically, optimizations are done using performance as an excuse. Before these types of modifications of the code are done, a requirement specifying the need for the change should exist.
Even through the practice of Test-Driven Development, it is possible to paint yourself into a corner...