Seat switching etiquette is – I think – fairly well established, at least here in the United States. You’re entitled to the seat on your boarding pass. If someone prefers a seat other than the one to which they’re assigned they should ask politely, but it’s entirely up to the person with the desired seat whether or not to switch. They can say no for any reason, or no reason.
If you want to switch seats it pays to have a generally desirable seat. Don’t offer me a bulkhead window seat in first class, or a middle seat by the lavatory in coach. If you have a poor seat and you’re asking to switch be extra nice, have an extra good reason, and even consider offering something (find a Coasian solution: offer your worse seat plus cash).