How to upgrade on iTunes if you already own the HD

Computer (PC): Click on the down arrow next to Buy and send a gift to your iTunes email

https://support.apple.com/en-us/HT210384 This is the link to download the standalone iTunes for Windows.

Code: Just redeem a code as normal

Mobile: Search for the movie. Click on the upper right hand corner arrow, then click on gift movie, send a gift to your iTunes email. The code will come by email and redeem it to your account:

As of 4/17/20 gifted movies port in 4K to MA if they are 4K on iTunes.  You can not use gift cards to gift a code. It takes from your main payment option. 

As of 4/23/21 If you gift a movie before it goes 4K on iTunes it will now port 4K to MA if you redeem after it goes 4K on iTunes. The previous issue where it ported HD has been fixed.

Bundle Upgrade Option from Zu Nim

How to get iTunes 4K upgrades using bundles

As of 12/22/2020 all bundles with 4K movies in them should port 4K. (The prior info of the $0.99 bundles not porting 4K has been fixed)