Vudu does exactly what you'd expect. If you own the SD or HD version of a title, you are given the option to purchase a better version right from the title's page. Straightforward.

Prime Video has separate listing for all their 4K content. This makes purchasing the 4K version of content very simple. Search Prime Video for the title and add "4k uhd" to the search terms and you should find the 4K entry for that title. If the title is available on Movies Anywhere, it should export as 4K.

If you already own a title at Microsoft, it doesn't normally allow purchase of a better resolution because no Buy button is available.

To upgrade you must use the Windows app. First, sign out. The Buy button will now be available for the resolution you want (HD or 4K). Click the Buy button, then sign in to complete the purchase.

If you own a title in SD, iTunes makes it easy to purchase the HD/4K version right from the title's page. If you purchased the HD version at iTunes and the title later is upgraded to 4K, you receive rights to the 4K version at iTunes; however, these new 4K rights do not export to Movies Anywhere (and therefore to no other retailers). If you purchased the HD version at another retailer and it was imported to iTunes from Movies Anywhere, you can't watch the title in 4K and you are denied the ability to repurchase it in 4K.

The only way around this is if you are able to purchase the title as part of a bundle. This will, at a minimum, give you 4K rights at iTunes itself. But generally it will not export those 4K rights to Movies Anywhere.

If you want 4K rights to export to Movies Anywhere you will need to purchase the bundle when its total price does not end in 99 cents. The reason for this is because normally a bundle of movies is sold in a single transaction and iTunes fails to export 4K rights when that happens. However, if the price ends in something other than 99 cents, that means the price of the individual movies in the bundle is less than the current bundle price. iTunes makes a separate transaction for each movie in the bundle (instead of a single transaction) and the 4K rights will also be exported to Movies Anywhere.

If you already own a title at FandangoNow, it doesn't normally allow purchase of a better resolution because no Buy button is available. There is a workaround on the web site that requires the use of a second account that does not own the title to upgrade.

Log into the secondary account. Begin the purchase process by clicking on the button associated with the resolution you want to purchase (HD or 4K). You will see the order summary screen. Do not attempt to purchase the title.

In a separate browser tab, visit the FandangoNow home page. Sign out of the secondary account and then sign in to your primary account. Close the tab and go back to the order summary tab, where you can complete your purchase.

Notes: The payment method you want to use must be available on both accounts. Either have the same credit card on both accounts, or use Visa Checkout. Mismatched discounts or credits may also foul up the workaround.

There is no current method to upgrade titles at Google Play. They suck.

There is no current method to upgrade titles at Xfinity.

There is no current method to upgrade titles at Verizon FiOS.