Hey Luke,
This is a really tricky one given that you’re trying to get the selected data of the select field. There’s no way the Prismic will remember this even if you keep the same API ID.
The only solution that I can see here is to use the import/export feature that starts on the medium plan.
You can export the current set up and run a script to get the select data and convert it in to key text fields. Then change your custom types and re-import the new restructured data.
Let me know if you have any further questions about this.