Some developers offer a "lifetime license" that includes the 2.0 upgrade; most do not. If you buy a premium theme today, ask the developer explicitly: "Will this entitle me to the MyBB 2.0 version?" If they hesitate, assume the answer is no. Yes—with caveats.
However, there is one universal complaint among MyBB administrators: mybb premium themes
If your forum is a casual group of 50 friends, a free theme is fine. If you are monetizing via ads, memberships, or e-commerce, a premium theme pays for itself in increased engagement within the first month. The elephant in the room is MyBB 2.0 (still in development). This will be a modern, MVC-based rewrite using Twig templates and a responsive core. When 2.0 releases, all current 1.8 premium themes will break completely. Some developers offer a "lifetime license" that includes