I've come to the conclusion that you can't architect yourself out of this.

What you might do is write tests early, so at least you have integrated a test rig to build on if the system actually gets any use.

Most of the work on mature systems is like this, rewriting and refactoring stuff. If it is very concrete, non-abstract, it's generally easier to work with in this phase than if it was cleverly and abstractly architected. Even if it's a big spaghett.