How Do You Fix a Legacy Code-base?

If your code-base does not conform to modern day quality standards and efficiency standards what do you do? Start over, isolate, or migrate?

Doing it over would allow you to do everything right but support would have to be dropped for most of your customers. That is suicide. Migration is a useless and demoralizing process that will leave most programmers (including this one) slipping back into bad practices and hacks to fit the existing methodology, in an attempt to get work done.

So with staring over being suicide and migration fixing essentially nothing, what do you do? Compromise, I guess. Isolate the existing code-base via and API and services giving the option to rewrite some essential pieces.

I guess that is the only option. *sigh*