Remember the restoration of the Ghent Altarpiece by Hubert and Jan van Eyck? It was pretty eye-opening at the time:
https://arstechnica.com/science/2020/07/new-restoration-reveals-painted-over-original-version-of-van-eyck-lamb-of-god/

Additional article including HD comparisons before/after restoration:
https://www.codart.nl/other/ghent-altarpiece-website-enhanced/

>From left to right: before restoration (with the sixteenth-century overpaint still present), during restoration (showing the van Eycks' original Lamb from 1432 before retouching), after retouching (the final result of the restoration)