Ron Jeffries, one of the three founders of eXtreme Programming, and one of original signatories of the Agile Manifesto states:
We must evolve the infrastructure. It’s not a rule, it’s worse. It’s essentially a law of nature.
Please read his whole excellent post to understand why. And then, if you haven’t already, read Martin Fowler‘s book Refactoring and start applying it.
Ray on Software Architecture 