I have done this a few times when I get a last minute F frequent flyer ticket and my husband is in B. I offer the person next to my husband the F ticket so we can sit together. The person has always been delighted. I do tell the FA so there is no confusion in F where they usually use your name.

The only issue I even had was when I offered my window F seat to a person in B class on a HKG -Vancouver flight . The opportunist person across the aisle in F immediately jumped into my window F seat and the person who was moving from B had to take his aisle seat which was less desireable. I was annoyed but too tired to intervene.