Hey there. Unfortunately, one of the required checkboxes you must tick before enabling cross-save explicitly states, 'even if that purchase happened before activation'. There is no way to switch the active account, except to deactivate cross-save and re-activate the correct way - waiting out all of the mandatory cooldowns. All Steam-owned DLC should be playable on Steam using the XBox characters, as they are tied to the platform you launch the game on, not the origin of the characters. However reward passes and dungeon keys are tied directly to the Steam characters, so those will not be available.