Public transportation - etiquette question

[stormkeeper_lovedoris]

So, let's say you get on a crowded train or bus, and you take a seat next to someone. As the train continues its journey, it gets un-crowded and empty seats become available; you could get up and move to one where you won't be sitting next to someone.

Is it okay to do that? I tend to stay where I am based on some strange belief that the person I'm sitting next to will be offended.