When setting an old TL based site setting in the console e.g.:
SiteSetting.min_trust_level_to_allow_ignore = TrustLevel[3]
We will silently convert this to the corresponding Group::AUTO_GROUP. And vice-versa, when we read the value on the old setting, we will automatically get the lowest trust level corresponding to the lowest auto group for the new setting in the database.
Martin Brennan