Stan Shinn has put together an interesting process for converting 1st edition Dungeons & Dragons monsters to the new 5th edition. He’s provided a PDF document you can download and use to quickly run through some conversions, as well as a video (embedded below) showing the process in use. He also shows the mathematics behind it and converts monsters through and back again to show how similar/different they are from the actual final versions that have been presented in official posted documents. As you can see there’s a fair amount of disparity in some of the results, but this may be a good thing as some indications are coming through that some 1st level monsters in the new edition are very tough during actual play.
Update: Stan has updated the tracker.