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Why don't you create new keys and let the old ones expire? The point with PGP is to be sure that the issuer is actually the real issuer.
Because I have the doubt that someone can use the old keys and that these people cannot then open the messages because it does not seem to me that Thunderbird accepts an old key.
I'm not completely sure what I wrote above, but I'm afraid it could happen since I haven't revoked the old keys.