Passenger Trains > changing boarding station on an e-ticket

Say I make a reservation now from BURbank to DAVis for Dec 22nd and get an e-ticket, but later decide I want to board the bus in Glendale instead. Is it possible to change ONLY the bus-to-Bakersfield portion online while leaving the rest of the trip as-is under the same reservation number and without unleashing additional costs? Or is it only possible to make the change by cancelling the original bus-train-bus reservation and making a new one ? (which one would not want to try close to the Dec 22nd travel date)

Call Amtrak and change the boarding point. If you do not, then bakersfield north space will cancel.

As I read the instructions, no fares change. But if you cancel the entire reservation, fare could go up on the Bakersfield north, under the yield management.