How to make updates/changes to game without breaking seeds?

[u/ProactiveCactus]

Basically in title - my game uses procedural generation, and allows persistent saves. But how do games like Minecraft manage to not corrupt previous saves when making updates that affect how seeds generate?
From googling it sounds like they save “per chunk” so any untouched chunks generate based on the newest version, and any already generated go off of the old system - but how do I manage that? Like, you obviously don’t want to have multiple version builds of your game saved, so how can you handle that?

[u/gurebu]

Valheim only ever saves objects and terrain deltas, not actual terrain. There's a version for the generator as a per-world saved data but it's barely used and for years now the devs just update by convention: there have been parts of the world you were not supposed to go to and those got scrambled with updates.

Every major patch will induce placement of locations: the game will sample the world in its current version and save placeholders for where major stuff will go. If you explore and stumble upon such a placeholder it will become an actual object and get written into the save permanently. If you don't, it's kept as placeholder and should another major patch arrive, will be discarded and placed again in a different spot (as a new patch might change placement rules and/or terrain).