In reading the above responses, some think that cancelling on the same day but after your originating flight has departed is ok and has a chance of getting one a full refund if the outbound flight is canceled. But if that's a risky strategy regarding getting even the taxes refunded, I think I'd go with cancelling before departure and making sure I at least got the taxes back.

If this would impact your ongoing connection to another form of transportation, it seems that you have an argument for cancelling right there.

Nope, it is the final leg on the return. In your experience, is it even worth a call to try to get a full refund for this relatively small schedule change?

Still looking for more input on whether it is better to do this several hours before the flight departs or possibly after the outbound departs, but on the same day?

Furthermore, I always seem to get folks with Indian accents of limited knowledge on the phone when I call United. Am I better off submiting the refund request online if I can?

But this may be a moot point - when I look at my reservation online it says I must first login, so I login, and I get the very same message anyway.
It looks like my bmi ff # is attached to the reservation at the moment, is that maybe why I am having problems working with the reservation on united.com ?