Originally Posted by
jsloan
There's nothing to be done until / unless your flight is cancelled.
If that happens prior to the day of travel, UA is responsible for re-accommodating you, or offering a refund. If it happens on the day of travel, it'd be Brussels' responsibility.
Keep in mind on the day of, Brussels support lines are going to be completely jammed. While Brussels is the one
obligated to assist you at that point, United
can assist you and on past experience there's a fair chance United will assist you, so try both in parallel to see which gets through first.