1. What is the route? If this is a departure from the EU, you are entitled to a refund of 75% of the base fare, Otherwise. all UA is obligated to do is to issue a refund for the fare difference. UA calculates this as the difference between the fare paid and the J fare as of the date of the downgrade. Often means that the refund is little or nothing. Nothing you need to deal with until after the trip.

2. UA is not obligated to change your flights, but will generally do so. This sounds as though the agent documented your PNR record denying the change, so you need to overcome the above. For this reason, I agree with above that you introduce your request with the change UA made.