Not sure what the big deal is. UA policy is to rebook within 300 miles in IRROPS and BTV-YUL is just over 100 miles. The entirely separate question is whether you will be permitted to board and that is a matter of proper documents, e.g. passport for US nationals.
Before you ask for rebooking, do take a look at the cost of your ticket and of a new YUL ticket. If the latter is significantly cheaper, buy a new ticket and take a refund on the BTV ticket.