The UA agent should have been willing to off-load you. That is your right not to fly on the flight and you could apply the ticket less the change fee to a future flight.

A second issue is whether United would be willing to endorse the ticket to AC, so it can be used on that carrier. They have no obligation to do so as this will result in reduced revenue for United. By the same token the E-ticket may be accepted by AC without the endorsement, but UA may not pay AC, depending on fare basis the endorsement may or may not be required. AC is taking a risk for revenue which may be acceptable to them if there are seats that would generate no revenue anyway.

Bottom line you did the right thing by just finding another agent.