I bought the plugin a few months back but can't seem to find the premium edition in my purchases? Is it a separate purchase from the plugin?
Viewing post in MV3D - 3D Rendering Plugin for RPG Maker MV comments
From what I can gather, it's literally nothing more than a few changes in the plugin. Look at the bottom of https://mv3d.cutievirus.com/documentation, it gives a brief rundown of the new commands there.
For example: "3D models can be imported using the model() function. For example, model(filename.obj) will load filename.obj from the models folder." - this command only works in the Premium plugin (mv3d-plugin 0.6.8.2), not the demo one(mv3d-plugin-demo 0.6.8.2)