I am not sure if this was possible with an older version of your plugin (but I guess, it was in version 1.5 according to http://wordpress.org/support/topic/plugin-post-expirator-move-it-to-another-category): In the options I can only set posts to draft or delete. But I want them to stay published, but in a specific category at the expiration date.
Before expiration date: Post is published in category A
After expiration date: Post is published in category B and removed from category A
Category B is the one I set in the options as Default Expiration Category.
Is this possible?