Hi Duncan,
This is an old thread at this stage and was in relation to a Slice Machine update, rather than legacy Slices.
For the moment, this still isn't possible. It's on the team's radar but there are still a few things to deal with before we can build a legacy migration tool.
There are no issues in terms of Slice's breaking in production and Slice Machine is out of beta now so it is considered stable.
I'm glad you're enjoying working with Slice Machine, the next thing on our radar is mocks for these content relationships.
Let me know if you have any other questions.
Thanks.