I believe this affects worlds from all previous versions, including older releases, snapshots, and pre-releases.
Ordinarily the game would warn you when you try to open an old world in a new version and ask if you're sure you want to update, but it doesn't. There's no exclamation point next to the play button, nor is there a screen to confirm your choice to update.
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While upgrading worlds to release versions is a supported feature, it would be nice to still get a prompt before you actually upgrade the world because it would save a lot of pain if you accidentally upgraded a world that you only play in 1.8 to 1.17. As there is no way to downgrade worlds, I would expect at least something preventing you from upgrading a world before you intend to.
edit: meant to put expect instead of except
Agreed with PrimePlatypus12
What would happen if worlds got corrupted when upgrading to 1.17?
Worlds we spend days, months or even years.
how is this WAI?