There is no difference between "epic" and "conventional epic." Let me explain. The epic originated as a poetic form and is governed by certain rules as to form, structure and style. For example, regarding setting, an epic setting must accord to this formal convention:
the SETTING is vast in scope, covering great nations, the world, or the universe (Auburn University)
A "convention" in literature is defined as a definitive and usual feature of a literary genre, such as the epic genre.
Thus, an epic is a specialized form of poetry or novel in the epic genre that accords with specialized conventions.
In other words, an epic is conventional (follows literary conventions defining it) in regards to form, structure and style. Thus an epic that conforms to the specialized requirements of form, structure and style is a "conventional epic" because it conforms with all epic conventions.