Update: thanks to @sween64, he has shared a list he found on reddit(?) with titles listed for upgrade and the source that confirms it.
I've seen people mentioning a lot that you can buy cheaper PS4 games and get a free upgrade. So I thought I'd bring together the definitive list (best I could find it) with amazon references below. Some companies (like Ubisoft) are offering upgrades to disc and discless version (but disc version require the disc versions of new consoles), whereas PlayStation haven't confirmed what the process looks like (to the best of my knowledge).
The biggest savings seems to be $10 on a handful of games.
|Assassin's Creed Valhalla||$69||$79|
|Dead By Daylight||$52.30||——-|
|Far Cry 6||$69||$79|
|Horizon Forbidden West||——-||——-|
|Kena: Bridge of Spirits||——-||——-|
|Marvel's Spider-Man Miles Morales||$79||$89
|Mortal Kombat 11||$29||——-|
|NBA 2K21 Mamba Forever*||$134.84||$139|
|Rainbow Six Siege||$19||——-|
|Sackboy A Big Adventure||$89|| $99
|The Elder Scrolls Online||$39.95||——-|
|The Witcher 3||$39||——-|
|Vampire: The Masquerade - Bloodlines 2||$69||——-|
|Watch Dogs Legion||$69||$79|
*NBA 2K21is a little strange (thanks @Phreakuency). You only get the upgrade if you get the Mamba Forever version. So though this is what I've used for comparison in the table, the base game on PS5 is actually only $88 and probably makes more sense to get if you don't care about the extra content.