Function migrate 

pub fn migrate(
    storage: &mut dyn Storage,
    old_keys: &[String],
    name: &str,
    warnings: &mut Vec<String>,
) -> Profiles

Makes the first profile, called name, and moves the player’s keys from before profiles (old_keys) into it, once.

Nothing is lost if it stops part-way: each key is copied (unless the profile has it already), then the list is saved, and only then are the old keys deleted. Until the list is saved, every launch tries again; once it is, the move is done, and an old key that couldn’t be deleted is a copy nobody reads.