Hey @jjames.home,
I did sign up to the upgrade tool early last week, and then the migration tool later last week but haven't received a response yet.
The Migration API is generally available. You can find instructions on how to use it by going to the Migrations tab of your repository.
You can opt in to the upgrade too by going to localhost:9999/labs with Slice Machine running.
So far the plan is
Your plan sounds good to me!
I think the only change on the live site would be to change document.data.body => document.data.slices
If you're referring to the Slice Machine migration, I'm not 100% sure since I haven't made the change before. It's possible that there will be other changes to the structure of the types and slices, but I'm not certain.
Sam