Update your settings

Rename a field or move a class

Use Unity's own [FormerlySerializedAs] and [MovedFrom] attributes; they work here as everywhere else. For changes they can't express, Settings Kit has two hooks, described below. Both run in the editor when settings load, so builds always get already-updated values.

Convert saved values

Say version 1 stored int volume from 0 to 100, and version 2 stores float masterVolume from 0 to 1. Implement IVersionedSettings, keep the old field under its old name so its saved value still loads, and raise Version:

[System.Serializable]
public sealed class AudioDefaults : BuildSettingsEntry<GamePage>, IVersionedSettings
{
    [SerializeField, Range(0f, 1f)] float masterVolume = 0.8f;

    // The version 1 field, under its old name so its saved value still loads.
    [SerializeField, HideInInspector] int volume = -1;

    public int Version => 2;

Then convert the old value in Migrate:

    public void Migrate(int fromVersion)
    {
        if (fromVersion < 2 && volume >= 0)
        {
            masterVolume = Mathf.Clamp01(volume / 100f);
            volume = -1;
        }
    }
}

Recover a deleted class

Say you replace a NetworkSettings class with NetworkConfig. In any class of your project, declare a struct with the old fields you want back. It doesn't need all of them:

public static class LegacySettingsMigrator
{
    [System.Serializable]
    struct OldNetworkSettings
    {
        public string address;
        public int portNumber;
    }

Then add a static method marked [SettingsMigrator]. It receives every saved setting whose class no longer exists: recognize yours by its name, read it with TryRead, and copy the values into your current settings.

    [SettingsMigrator]
    static void RecoverNetworkSettings(MissingSettings missing)
    {
        if (missing.ClassName != "NetworkSettings"
            || missing.Namespace != typeof(NetworkConfig).Namespace
            || missing.AssemblyName != typeof(NetworkConfig).Assembly.GetName().Name)
            return;

        if (missing.TryRead(out OldNetworkSettings old) == false)
            return;

        NetworkConfig current = Settings<NetworkConfig>.Instance;
        // ... copy old.address / old.portNumber into current ...
        Settings.Save();
    }
}