I have no idea.
However, this may work because it did in the past :
DH and I went to Las Vegas last month. Our return seats were separated on the plane. I went to the ticket agents at the gate of our departing plane, and they were able to fix our seats AND print us our return boarding passes at no charge.
I plan on asking the agents for our return boarding passes (if possible) on our departing day again.
It may work, it may not. But I'm gonna give it a shot nonetheless. It will save us time returning, for sure! Plus the hassle of printing the passes from the airport on departing day.