RichieD is correct, but to be a little more precise, it's more like there's no guarantee an older version will be able to read files saved with a newer version. In fact, if object structures aren't changed, there's a good possibility an older version will read files from a newer version. For example, version 3.3 should happily read files from version 3.3.1, because only the "launcher" was changed. Unfortunately this is more the exception than the rule.