I am going to ask someone if they want to switch seats with me.
What is your reason for wanting to switch seats with them?
Because they're blocking my line of sight.
It can be annoying when someone blocks your vision, but you can't order them to change seats. If you demand that somebody changes seats with you, they are likely to be irritated and may not comply. Instead, ask nicely and hope that they'll be considerate.
I'm not ordering. I'm going to tell them.
Well it would probably be better to kindly ask them if they do not mind switching seats. If you demand for them to switch seats with you odds are they are not going to want to do that since you are asking so rudely.