You might consider whether you would follow this route. Dirty upgrade to /e/OS-T look possible!
The official advice is to treat the Upgrade as a new install Install /e/OS on a OnePlus 8T - “kebab” (as apparently it has been reported that users of certain devices bricked their phones).
Please read the thread carefully to understand why it may work for you.I suggest you ask any questions there. Always prepare with a backup of essentials before undertaking any intervention.