Dr. Nikolaus Klepp wrote:

> Well, the receiver of the mails wants to use a new keypair for all new
> mails. No matter how careful he is, it ends either in reusing the old
> private key occasionally or in deleting the old private key (and loosing
> all old mails of douptful value).
> 

you can remove the trust from the old key and it will not be accepted by
gpgme for encrypting mails, even if you have same name, you will have one
key that is probably white or red and one key that is green. you use the
green to encrypt, the white gets rejected.

> In my case, I like to create a keypair per project. When the project is
> "done", I' like to archive the mails in a readable (aka unencrypted)
> manner in a save (offline) place and delete the keypair.

It would be enough to revoke the trust, but keep the key to be able to
decrypt in future.
I don't see issue with this approach. Everything else would be developing a
solution for your use case(s).

This is my personal opinion, so it might be there are smarter heads around
to give us better ideas.

regards