A custom tracking host no longer has to sit under your sending domain. If you send from send.acme.com, your links can use track.acme.com, so recipients see your main domain.
What you need to do. Nothing for tracking you already run. To move it, enable custom tracking again with a parent, then publish the two DNS records Samva shows. New sends switch once the host is active.
samva email domains custom-tracking enable dom_abc123 --label track --parent acme.com
Custom tracking is included on the plans that list it.