Snapshot tests may be some kind of help here - if you take snapshots of some parts of your app before the change, and compare them to the snapshots made after the change, you’ll have a clear view what will have changed. It won’t help in the transition itself, but it’ll help you clean up after it.
As to the transition - have you considered changing the implementation of the old component, instead of creating a new one from scratch? It can go smoothly if you keep the component’s interface backwards compatible. Of course, it’s not possible in case of more thorough transitions.