An excerpt from the GNU Privacy Guide[1]:

    A key's expiration time is associated with the key's
    self-signature. The expiration time is updated by deleting the
    old self-signature and adding a new self-signature. Since
    correspondents will not have deleted the old self-signature,
    they will see an additional self-signature on the key when they
    update their copy of your key. The latest self-signature takes
    precedence, however, so all correspondents will unambiguously
    know the expiration times of your keys.

With that in mind, you could probably re-sign the key with a new
expiration date.  I'm not sure what "proper etiquette" is, but it seems
that doing so should be sufficient.

References
---------
1. http://www.gnupg.org/gph/en/manual.html#AEN329

-- 
Chad Walstrom <chewie at wookimus.net>                 | a.k.a. ^chewie
http://www.wookimus.net/                            | s.k.a. gunnarr
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