A couple of years ago I allowed PNG files to be posted, thinking storage was cheap and lossless was ideal for our site.

Turns out Windows pastes as PNG and the size of my storage has almost doubled in two years compared to the past 10 years.

Ideally, I could convert these PNGs en masse to JPG and then somehow update the database to reflect the new extension on the affected files.

Has anyone done this or seen a post how?

There is no way in which to do this within the software. You would need a developer to assist you with something like that really. Ensure if you do try anything, that you backup fully before you start