Hi Colin,
6.2r5 and 6.3r21 are signed with different image signing keys. So, you will have to either update or delete the key before upgrading to r21.
Details here:
https://kb.juniper.net/InfoCenter/index?page=content&id=TSB16495
https://kb.juniper.net/InfoCenter/index?page=content&id=TSB16496