If they are not, the following changes can make a saved game unloadable:

1) a library/extension/adventure is upgraded past API25. Prior to this version, all loaded libraries/extensions/adventures automatically have the compatibility types library loaded
2) a library/extension/adventure is upgraded past API53. Prior to this version, all loaded libraries/extensions/adventures automatically have the compatibility unids API53 library loaded
3) a library/extension/adventure is updated and removes a library dependency

Libraries like extensions need to be checked for in the savegame and loaded as appropriate.