Looking to the future isn't the only plus of writing unit tests for old code. This afternoon I was finishing up unit tests on my animation class and I found a nice bug around trying to load corrupt animations. I never ran into the bug while coding my last game but this is the type of bug that you can hit in the future and spend hours trying to debug.
I also finished refactoring my manager classes to accept a mutex object in the constructor instead of creating platform specific manager classes. Once again I had tests ready to go after my refactoring was done that I could run to make sure I hadn't messed something up by accident.
I should also clarify that you don't really need to follow pure test driven methods where you create the test before you create the object. A lot of people find that technique ideal but I think you still gain a lot by having unit tests after the fact.