The new signing keys were released some years ago. For the upgrades you simply download the new key and upload this first to the ScreenOS device. then perform the upgrade itself after this and all is well. The details are in these articles.
Signing Key Articles
http://kb.juniper.net/InfoCenter/index?page=content&id=TSB16495
http://kb.juniper.net/InfoCenter/index?page=content&id=TSB16496