http://realorfake4k.com is a good place to consider if you are actually buying 4k or marketing puff.
This is getting a bit silly now. While I agree, it is annoying that more "true" 4K films aren't being made/mastered/released do you honestly believe that these re-releases from 2K DI are just "marketing puff"? If you need any evidence at all go watch Pacific Rim on Blu-ray then immediately after (or better yet side by side them) watch the 4K version the difference is night and day. UHD in my opinion is 25% about the resolution and 75% to do with HDR/WCG.
What I'm trying to say is; most companies aren't just putting the Blu-ray copy upscaled on to the UHD disc, they are properly going back and transcoding them with more information which gives us better detail and remastering the colours completely.
In other words don't just think "oh it was only a 2K DI so it must be rubbish, I'll just stick with my BR copy" that is crazy and you'd miss out on some of the best looking movies to come out on the platform so far.