Switch 2 backwards compatibility may become more complicated than expected. While original Switch games should boot on the new console, Nintendo referenced Switch 2 Edition titles. These games might take full advantage of upgraded Switch 2 specs but could incur added costs.https://www.notebookcheck.net/Nintendo-mentioning-Switch-2-Edition-Games-has-players-wondering-about-upgrade-costs.988779.0.html
Pure speculation here:
Of COURSE if Nintendo OR a 2nd or 3rd party studio ENHANCES and RELEASES a game for Switch2 it will NOT BE FREE but rather COST players who want the new version.
While the OLD version MAY play better on the S2 than the S1, "automatic game upgrades" is a pretty far fetched DREAM.