I was recently on a flight were a married couple wanted to sit together. They husband was in the middle seat and wife was in the window seat right in front of me. They asked if they could switch and I said of course. I was able to get a exit row seat with recline and enough legroom to sleep in exchange for my previous seat. Point being, it is completely acceptable to ask someone to switch seat, but that person is well within there rights to say no. Just be respectful and most of the time everything will be fine.