When a world is loaded from a previous version, Minecraft normally prompts a warning with the option to create a backup, however this is no longer the case for 1.17. In the past, smaller updates to world generation such as 1.14 to 1.15 would prompt a warning, but this is no longer happening for worlds being loaded into 1.17.
To reproduce:
Create a world in a previous version before 1.17
Load the older world in 1.17
There will be no warning, and the game will instantly convert the world to 1.17.
Attached is a video of loading a 1.14.4 world in 1.17.
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We're tracking this issue in MC-228166, so this ticket is being resolved and linked as a duplicate.
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Duplicate of MC-227787, which was later resolved as "Works As Intended".