Quoting Chad Walstrom (chewie at wookimus.net):
> IIRC, you create a new one.  Basically, once the expire time has passed,
> you cannot be certain that the original author has control of the key.
> You can edit the expire time BEFORE your key expires, which is
> interpreted to be a safe operation by others, they can assume that the
> original author is still in control of the key.
> 
> I think your SOL with that key, though.  Time for a new one.

I think gpg PREVENTS you from changing the expire once the key is expired.

$ gpg --edit-key tanner at real-time.com

<notice there is NO expire command>

pub  1024D/93D97DA3  created: 2001-01-15 expires: 2002-01-15 trust: -/e
sub  2048g/DB44054A  created: 2001-01-15 expires: 2002-01-15
<snip>
Command> help
quit       quit this menu
save       save and quit
help       show this help
fpr        show fingerprint
list       list key and user IDs
uid        select user ID N
key        select secondary key N
check      list signatures
sign       sign the key
lsign      sign the key locally
nrsign     sign the key non-revocably
nrlsign    sign the key locally and non-revocably
deluid     delete user ID
delkey     delete a secondary key
delsig     delete signatures
pref       list preferences (expert)
showpref   list preferences (verbose)
trust      change the ownertrust
revsig     revoke signatures
disable    disable a key
enable     enable a key
showphoto  show photo ID



-- 
Bob Tanner <tanner at real-time.com>         | Phone : (952)943-8700
http://www.mn-linux.org, Minnesota, Linux | Fax   : (952)943-8500
http://www.tcwug.org, Minnesota, Wireless | Coding isn't a crime. 
Fingerprint: 02E0 2734 A1A1 DBA1 0E15  623D 0036 7327 93D9 7DA3