Bottom line question: Can I upgrade from a WGA or AT fare to BS fare at the gate and pay the difference with a LUV voucher?

Background: As we near year end, I have not traveled as much recently and have only two more trips booked--a short r/t on an AT fare and a longer r/t on a WGA fare. I am interested in upgrading both trips to BS fares in order to renew AL+. Because these trips are for work, they were booked though Concur and thus I cannot upgrade these fares to BS in advance without raising a red flag in our booking system. (Company policy prohibits booking "business class" for domestic travel.) In the past, I have paid with a credit card at the gate to upgrade flights from AT to BS for mileage accrual purposes, which does not seem to get passed back to Concur--I get reimbursed for the WGA or AT fare, whatever was originally booked, and I pay out of pocket for the upgrade, which is fine. I happen to have a large LUV voucher (that I obtained recently in an oversell situation) that is more than adequate to cover the fare differential to upgrade these trips to BS. I would like to use it for that purpose, but am not sure if I can use a LUV voucher at the gate to upgrade my fare. I appreciate any insights or experiences.