updating a key's self-signature

[vedaal at hush.com]

i have two keys that i use extensively for e-mailing

one is a v4 rsa key (my default key), and the other is a v3 rsa key 
(for those correspondents who insist on or prefer the old key)

both were signed with md5 when they were generated years ago

when i try to sign them now, gnupg (1.4.2) prompts me to sign with 
a dh/dsa test key that is in my keyring, instead of with my default 
key

(i tried using updpref sha256 first, which was accepted,
but still couldn't sign a key with my default key)

is there any way i can self-sign them with a sha256 sig,
or sign them with my default key with a sha256 sig

(if not, can this be a feature request?

as signature hashing algorithms become less trusted,
but while the key itself is still trusted,
wouldn't it make more sense to be able to update the self-sig
rather than have to generate a new key? )

TIA,

vedaal



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