Had a UA flight cancelled and rebooked to something 5 hours later, agent would only offer credit and said for basic economy there is no residual value voucher given for rebooking to a cheaper ticket. She said that I would be able to request a refund after 12 months if I decide not to use the credit...however I don't see that published anywhere and if I wait 12 months I won't be able to charge back anymore....wondering if just filing a chargeback is the best thing to do at this point.