Deltarune - Not sure if this is being categorized as a patch, but if you buy it on Switch 1 you can upgrade to the Switch 2 version for free.

Rune Factory: Guardians of Azuma - $10 upgrade pack. Interestingly, the Switch 2 physical is a special cart that can boot the Switch 1 version on legacy hardware, or the bundled upgrade pack on Switch 2.

I believe those are the only two so far, and they're cross-gen launch titles rather than any older releases. Sega on the other hand is notably not doing this, they're porting a few of their Switch 1 titles at full price with no upgrade option.