Update to CoreELEC directly from LibreELEC?

[Sholander]

@Morguito

Or, if it’s simpler, which folder should I backup (and then replace) from CoreELEC to retain my current configuration in a brand new direct install?

You can use internal CoreElec option to make a backup. Copy this backup to some safe place, make a fresh install on an eMMC and when the install finishes, restore your backup.