Just an update on the flight, at the gate they told me it was impossible to do so. However, I checked in with head FA on the plane and she said between meal services it would be OK to swap. After a couple hours into the flight my wife took my seat (in a half empty J no less) to get some sleep, the FAs were very kind to her. I understand they are very strict with these kinds of things (due to some U.S. rule?) but was glad they could accommodate us.