How to extend subkey's expiry date?

[Adam Funk]

> > $ gpg --edit-key e3c5ee5e
> > ...
> > pub  1024D/E3C5EE5E  created: 2002-02-19  expires: 2007-03-07  usage:
> > CS trust: ultimate      validity: ultimate
> > sub  2048g/66796190  created: 2002-02-19  expired: 2005-02-18  usage:
> > E ...
> >
> > I've tried using "expire sub" and "expire 66796190" commands inside
> > edit-key, but I can't change the expiry of the subkey.
>
> "key 1" then "expire" should work.

Is it worthwhile to replace and revoke subkeys periodically, or should I 
just renew the expiry date?