Not usually. While the text does suggest that it is difficult to keep in mind which is which and that Claudius actually mixes them up (and is corrected by Gertrude), they are not normally played as actually interchangeable. Rosencrantz is often played by a smaller darker actor and Guildenstern by a taller fairer one, but this is not supported or contradicted by the text.
One might switch actors as you suggest but it is difficult to know what dramatic affect one might hope to achieve thereby.
Rosencrantz and Guildenstern are Hamlet's childhood friends. Claudius sends them to spy on Hamlet.
They are friends of Hamlet's from school.
rosencrantz and guildenstern
Guildenstern and Rosencrantz
Claudius and Gertrude set Rosencrantz and Guildenstern to spy on Hamlet to discover the cause of his apparent madness
He says it is a prison.
Rosencrantz an Guildenstern
True. Hamlet changed the king's orders to the English from "Kill Hamlet" to "Kill Rosencrantz and Guildenstern." He didn't have to do that; he could have changed the orders to "Give Hamlet some flowers".
Hamlet changes the letter going to the King of England to kill him when he gets there to say to kill the people who give you this letter,which were Rosencrantz and Guildenstern. So they were killed instead.
The royal couple are, in effect, recruiting Hamlet's old friends, Rosencrantz and Guildenstern, to spy on him for them.
Rosencrantz and Guildenstern are old school buddies of Hamlet's. They have been hired by the king to spy on Hamlet to find out why he is acting so peculiarly.
They are. If they weren't, Hamlet wouldn't have sent them to their death. They had the opportunity to betray him when they were out of his sight. It seems the king didn't completely take them into his confidence, though, so perhaps their loyalty wasn't total.