Well, I'm not familiar with your speaker cabinets, but any time yo start a DIY project like this you need to be prepared to go down unintentede rabbit holes :)
When to replace old drivers?
I have speakers with 15+ year-old Fostex drivers. They sound fine, but the paper is discolored and one whizzer slightly crumpled (child's prying finger years ago)... At what point (shy of obvious failure like buzzing) would putting in new Fostex drivers constitute an upgrade?