1: Do nothing. People will have to edit past broken images and use different image-hosting services going forward.
2: Make it so that all past imgur embedded images show up as a clickable link instead of as an embedded image, as if the poster had just posted the imgur link instead of using the [img] tag. I could also maybe make it so that if you click the link, it will JavaScript-expand into an embedded image (using the specified width etc.) without going through the image proxy.
3: Exempt imgur links from the image proxy so they all just work. But this has privacy implications, since imgur will see forum users' IPs. Also, imgur has apparently banned a lot of Tor exit nodes etc. as well, so imgur images won't work for everyone.
4: Exempt imgur links from the image proxy as above, but prevent using imgur embedded images in any future posts, to sort of phase it out.
We never know what additional restrictions Imgur may impose on us and therefore, I believe that we should avoid using it in our future posts.
My ideal choice would be to exempt the previous posts and restrict the use of Imgur for future posts.
Fortunately, we have Talkimg by Joker_Josue, which is an excellent alternative for hosting images.
Additionally, the script by TryNinja has helped many of us to replace our existing images with the ones hosted on Talkimg.
So we do have that option as well for the previous posts.