Old thralls that were placed before the update rolled are intentionally less powerful and they won’t work with the new level system. They were kept just so players have time to retrieve their items and replace them, as they’re severely underpowered in comparison to a fully leveled thrall.
There are no plans to tweak or adjust legacy followers.
You can find more information in this post which was created prior to the launch to provide further information on the new system and answer some of the most asked questions in the forums: