Google Play Movies is now upgrading past purchases, either SD or HD to 4K UHD.
Quote Movies you own will automatically be upgraded to 4K, on us: When 4K titles are available from participating Hollywood studios, we’ll upgrade your past movie purchases so you can stream in 4K, even if you originally bought the movie in SD or HD. It’s all on us, just open the Play Movies & TV app and we’ll let you know which titles have been upgraded.
Participating Studios include Fox, Lionsgate, NBC Universal, Sony (US only) and Warner Brothers.
So no Disney or Paramount so far.
From the wording, they may be similar to iTunes and not upgrading movies imported through MoviesAnywhere.
We’ll have to see if movies in your existing library that are upgraded get exported back out to MA as 4K.
Quote Watch movies you’ve already bought in 4K
Some SD and HD movies that you’ve already bought or redeemed might be upgraded to 4K. To be updated to 4K, movies must be:
Bought or redeemed from Google Play or YouTube.
Only SD movies bought or redeemed before October 23, 2018 will be upgraded.
From Fox, Lionsgate, NBC Universal, Sony (US only), or Warner Bros.
Available in 4K, in your location.
Note: Upgraded Family Library purchases can be shared at 4K quality.