@dan.parker, a quick update on the migration path: the team is planing to define a strategy for the transition path and the specs for migrating existing Legacy repositories to Slice Machine, but we don't expect to have any official guide yet.
In the meantime, we're keen to connect with the community to build the best transition path with the help of users. We're hosting a Product Meetup on the 28th of July where we'll discuss the Slice Machine migration topic, we'll have guests who'll share their experience of Migrating existing sites to Slice Machine. You can subscribe here: Product Meetup by Prismic | Prismic
I hope this helps 