restructuring tables and model relationships in order to use STI

Hey James! I'm going back to the garage app and want to implement some of the things I learned from you over the last couple of weeks. As a first step, I'm restructuring the models and databases. Planning to use STI for Users (with Customers and Employees inheriting from User) and for Repairs, with Invoices and Estimates inheriting from Repairs.
Ah! If any of this code was in production, you couldn't delete these migrations. You'd have to build on top of existing ones.

How would you instead do this work by only modifying existing tables, using brand new migrations, without touching any old migrations? Admittedly, it's not important in the early days of your own project, but it would be better practice to learn how to do this by adding to an existing system with data which you don't want to lose.