Instead of reading from raw state, migrateState can takes an argument and @migrate({}) does deserialization for it. e.g.:

    migrateState({messages}: {messages: Vector<OldMessageFormat>}): Vector<NewMessageFormat> {
        let old_messages = messages;
        // same as your implementation below

Why should we pass old messages?
I think that the whole idea behind migration is to rewrite the data in new format. It means:

• read the old data
• reformat it
• save new data to state

Your approach works and looks intuitive! I was thinking a syntax sugar to read the old data to avoid JSON.parse(near.storageRead("STATE")) pattern. User can write:

@Migrate
migrateState(oldState: OldStateFormat): NewStateFormat {
    let newState: NewStateFormat = some_reformat_steps(oldState);
    return newState;
}

@Migrate would know how to load (deserialize) oldState given OldStateFormat. @Migrate would also serialize reformat state using newStateFormat.

A possible @Migrates's deserialize:

let oldRaw = near.storageRead("STATE")
let old = OldStateFormat.deserialize ? OldStateFormat.deserialize(oldRaw) 
  : OldStateFormat.reconstruct ? OldStateFormat.reconstruct(JSON.parse(oldRaw))
  : JSON.parse(oldRaw)

^ is i think nice to have in API but it's up to your preference. The PR is considered done!

Since we do not have any NEP - or complex approaches are questionable. Let's get back to it if necessary.