I uploaded thousands of 12 MP - 16 MP photos to Google photos using the original application, Google Photos Backup. At the time, the max quality for these photos was around 3 MP. Is there a way to force a re-upload of my 3 MP photos to take advantage of the new larger max resolution of 16 MP?
You have two options as far as I can tell:
Select all your images and delete them from Google Photos. Note that this will remove the photos from all synced devices, so you'll want to uninstall the Photos app from your phone/computer first.
Then, reinstall the app.
(note, I've never tested this so I can't vouch for its efficacy. But it seems like it should work)
Manually Upload All The Things
I have, on a few occasions, forgotten that I've uploaded some image already. So I've manually uploaded it through the browser and that's resulted in duplicate images. So you could do this with your own images - I'd probably copy them to a computer first, naturally.
Though then you'll be stuck with a higher and lower res version of the file, until you delete all the lower res versions. I couldn't find a way to search by file size or upload date (e.g. "uploaded before 2016-05") in Google Photos, though.
Ensure You've Backed Up!!!
Note that especially with the first option there's a potential for Google to delete all of your photos from all of your devices. You should back your photos up to some sort of read-only media, like a few DVDs or several CDs or something. Double check that you're able to access all the photos (especially those you care about).