This question already has an answer here:
I missed expiration date of my
gpg key and have to generate a new one afterwards. Now I want to establish trust between my old and new key.
Naive call like
gpg --local-user DE5A457C --sign-key BCBE2B4A
ends in error message
gpg: skipped "DE5A457C": unusable secret key
How do I force
gpg to sign a new key with an old, expired one?