Currently the downscaled and upscaled tags are being used in opposite ways at the same time.
Since examples are better than walls of text:
posts with "resized"+"downscaled" and smaller source (this happens when the artist makes the version at the source smaller, which happens more often than one would think):
post #2777476
posts with "resized"+"downscaled" and larger source:
post #2876323
post #2759496
posts with "resized"+"upscaled" and smaller source:
post #2916620
post #2877972
post #2934134
posts with "upscaled+waifu2x":
post #2832559
post #2812855
Shouldn't downscaled_revision (plus bad_revision where relevant) be used instead in cases where the artist edits the source afterwards? In cases where the image on danbooru is smaller I'm not sure how it could be handled, as it may happen that the artist makes a larger file available at a later date - you can't really be sure whether it's an upscale or a better version, so perhaps just using md5_mismatch with no resized variants would be best.
It doesn't make much sense to have tags that are used at the same time in two completely opposite and contrasting ways.
Pinging @BrokenEagle98 as this discussion started in the Danbooru Discord, after I noticed his bot adding upscaled to some posts with valid danbooru versions but sources that were downscaled by the artists afterward. He referred me to topic #13591 but it seems his questions regarding this in that thread were completely ignored, so perhaps a new thread will get some definitive answers on the matter.
Updated