Backwards compatibility is like... 99% likely at this point. Both physical and digital.
I think Switch games will use the extra power of the Switch 2 by default. That means:
1. If they use dynamic resolution, it will be more stable and keep at the higher number most of the time (example: Xenoblade 2 runs at a dynamic 360-720p resolution. On Switch 2, it will probably stay at 720p a lot more often, if not at all times).
2. Framerates will be more stable. Games like, say, Bayonetta 3, which aims at 60fps, but consistently falls under 50 or even 40 fps, will probably reach 60 more easily. Games like Age of Calamity will finally run at stable 30 instead of bouncing between 30 and 12 (yes, 12) fps.
3. Loading times, popping and texture loading will probably improve as well. I imagine that games like Batman Arkham Knight and Hogwarts Legacy will GREATLY benefit from this.
Again, this would be just Switch 2 running Switch games without any extra changes. But there's always the possibility that developers decide to update Switch games to take more advantage of the new hardware. Maybe third party games like DOOM, Wolfenstein or The Witcher 3 will get patches to add 60fps options. Who knows.
And obviously, we could also see straight up "remasters" that completely take advantage of the Switch 2 hardware. All of the games above would benefit immensely form this, although it could also be lazy attempts to cash grab.
