What updates would cause old save files not work in the updated version?

Rekize

Hello, I am using Unity 2019.4.6f1 and AC 1.72.4.

Usually, the save files saved before updating the game would work normally after the build is updated.
But I found that sometimes loading the old save files after updating the game would mess up the game.

May I ask what kinds of update would make the old save files unuseable?
Particularly:

a. Adding or removing scene from the build settings. Including the case that:
Added a new scene -> build -> then realized the new scene is not a good idea and deleted it -> build again.
Would this cause the old save files that are saved before adding the new scene not work even after the new scene is deleted?

b. Adding or removing variables (global, local, or component) to the game?

c. Any other updates I should be aware of?

Thank you very much for your help.

ChrisIceBox

Adding or removing scene from the build settings. Including the case that:

Added a new scene -> build -> then realized the new scene is not a good idea and deleted it -> build again. Would this cause the old save files that are saved before adding the new scene not work even after the new scene is deleted?

If the scene structure was put back to how it was at the time of the save file's recording, that should be fine.

The main thing to look out for here is the Reference scenes by field in the Settings Manager. If this is set to Number (i.e. Build Index number), then you need to avoid restructuring your build scenes, because save files will refer to scenes by their build index number. You can add scenes, but don't remove scenes outright. If it's for filesize reasons and need to remove a scene, replace it with an empty scene file so that the build indices remain consistent.

Adding or removing variables (global, local, or component) to the game?

If you remove a variable that was referenced in a save-file, then the game should still load, but whatever data was associated with that variable will not be applied anywhere.

If instead you add a variable, then that variable's value will not be updated upon loading the old save. If the scene reloads as a result of loading, then unreferenced Component and Local variables will revert back to their default values, but Global variables will remain as they were. A custom script can revert them back to their default values before loading the save-file, if needed.

Any other updates I should be aware of?

The same is generally true for scene data, too - if a Remember component was since added, then it will not be affected when the save is loaded. However, if you check Always reload scene when loading a save file?, then scenes will always be set back to their default state before the save's data is applied.