Unlike many ancient structures, the Pantheon has mostly been left intact across the millennia. The Dome is entirely original and so is most of the remaining structure. From what I know, the majority of the restoration and renovation work has been to the interior ground-level walls and some of the exterior along the ground level, but even these are mostly minor renovations. In this case, what you see truly is for the most part what always was, and especially for the main awe-inspiring part, the great dome.
I don't believe the dome structure was radically altered across time. It's possible I misunderstood but my belief was it was only cosmetic changes to the dome itself. Other structural changes were made to the portico and doorways.
The Pantheon is very notable among Roman monuments for having little done over the years other than cosmetic upgrades.
of the original Pantheon from 27 B.C. very little remained.
It was rebuilt after the fires of 80 A.C. and 110 A.C. damaged the original structure.