I can't stop encryption being done with a wrong key

[Anne Wilson]

I have a friend whose gpg key became corrupt.  He created a new key, and I 
imported it.  Then we discovered that KMail insists on trying to encrypt using 
the old key, even though I have changed his addressbook entry to reflect the 
new key.

At this point we thought it was a KMail issue, so I moved to Thunderbird for 
answering his mail.  Signed mail in both directions is no problem.  He can 
send an encrypted message and I can read it.  The new key is fine.  However, 
when I send an encrypted message to him we hit the rocks.

In Thunderbird I have only a minimal addressbook.  I set his record to use the 
new key for encryption, and I can't see any way that Thunderbird should know 
about the old key.  However, the test email I sent him was signed by the RSA 
subkey of his old key.

Can someone please explain to me how this could be happening, and what I need 
to do to correct it?  Should I remove his old key from my keyring?  If I do, I 
assume that I won't be able to read his older messages.

Thanks

Anne
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