Google Play Movies is now upgrading past purchases, either SD or HD to 4K UHD.
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.
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.