My own DVD/Blu-ray collection has been in a state of near-constant flux for years. I've mostly been replacing old anime titles on DVD with their Blu-ray equivalents; I don't often do that for live-action movies nowadays because I've replaced most of them with Blu-ray versions by now. (I also have a small selection of HD-DVDs which I have long since replaced with Blu-ray versions, but I still have the original discs, not knowing where to send them.) All my new material is on Blu-ray except for certain anime titles that aren't being made that way (and may never be at this rate, anyway). I can't afford to upgrade to Ultra HD yet because of the cost factors, and even then I may not replace everything with 4K versions, as it's just too much of a problem to face financially. (I do have one title in 4K, Guardians of the Galaxy Vol. 2, which came with the Best Buy Steelbook that I got for the 3D version, but it may be one of the random defective copies.) So I do what I can when I'm able to, with my library.