You don't need the old version, if by old version you mean the files for the version that you are using. You can do a dry run of the upgrade, or a trial upgrade if you will, without disrupting your live forum by duplicating your main live forum to a test folder or a subdomain and proceed with upgrading that to the latest version and see how things will go. If you do that, you won't need to decrypt anything.
That being said, can you please elaborate a little more about the issues that you have been facing as per your post quoted below?