Add new choice to existing site column created by feature

Hi all,

I've created a Choice site column through a feature, now I need to add a new choice to it.

I've tried editing the feature and removing/adding it from my site collections but choices are not updated.

How can I upgrade those choices?

Thanks

Maurizio

<Field ID="{7970cd99-7493-4a94-b6ac-e1042f5a66ac}" Name="StatoDocumento" DisplayName="Stato documento" Type="Choice" Required="FALSE"><CHOICES><CHOICE>-</CHOICE><CHOICE>In elaborazione</CHOICE><CHOICE>Completato</CHOICE><CHOICE>Spedito</CHOICE><CHOICE>Superato</CHOICE></CHOICES><Default>-</Default></Field>