You don't need old versions of the files for recovery. All the instructions needed to use the new files are in this kb article.
https://kb.juniper.net/InfoCenter/index?page=content&id=TSB16495
If you have a device currently booting, you follow the first instructions to upload the new signing key and proceed with the upgrade.
If you are getting the bogus image error you follow the instructions near the bottom of the kb article to recover from that message with the new files.