Seeing your name, I now see that my title is incorrect. Should be something like. “If for any reason you cant use lamdera yet, would SurrealDB be the next best thing for elm-apps?” 
I guess nothing beats the lamdera experience when it fits your requirements.
Great question, I have not looked into migrations yet. ( I just discovered surrealDB 6days ago )
Found a discussion on discord about migrations:
Where the answer from one of the makers is this:
So no tools for migrations yet. But I guess one could turn of schema for the effected tables, change data and turn on new schema? But not sure if that is possible in a single transaction, have not tried.
Or map the complete db or tables over to a new db/table with transform functions, immutable style.
Talking about immutability:
I forgot to mention one of the biggest difference to other databases: SurrealDB is immutable. It is meant to tell you how a single field in a table has changed over time in a performant way and with compact storage of changes.
For those interested I found this really interesting whitepaper written by one of the authors comparing performant immutable database structures: (it is more or less describing design decisions and how the previous version worked under the hood, it had a different name then and was written in go. They have later found even more performant structures and changed to rust + renamed to surrealDB after that)