they don't care about old comb. why swap brood boxes if all is going well? if all your brood looks like that i'd change queens. what does it look like in the bottom box?
did you check closely to make sure she's not laying where bees have hatched out. it's hard to tell from the pic, but i think i see a couple of eggs and larvae. you may have pulled that frame right after a hatching and she's gone back and filled in.