Code maintenance and good design are different tasks.

New maintenance programmers often want elegant code, leading to the "let's just redo it all" idea. That's often much harder than just getting the new feature in place.

When I add a feature to existing code I often use the sprout object (or method or function) technique:

1. read the legacy code and find where the changes will go
2. add tests around that area of the legacy code, so that you'll more easily know if you broke something (with as little change to the legacy code as possible). This also allows you to know what variables are involved. This is the hardest part.
3. develop the new code, using test driven methods. You feed your new code the values from the parameters found in the 2nd step. Your new code is of course elegant and you develop it independently of the legacy code.
4. alter the legacy code so that it uses the new code. Cross your fingers and run the tests from the second step.