Good article - we've helped tons of teams complete long-term refactors at my startup (grit.io) and this approach definitely works better than attempting any big bang migrations.

Having a small and representative example PR is huge for both demonstrating how to do the change and for getting others on board.

The one thing I would add is to set up some basic progress monitoring (cases/files remaining). It's motivating to have a graph tracking progress over time, especially when building momentum early on.