As you ask what is essentially a question of good etiquette, I can only give you my opinion; which is simply guided by the fact the aircraft is a communal space and our actions should be taken to minimize affecting adversely those around us. Other posters here I am sure will have more robust views on what problems attend exclusively to unfortunate third parties. (i.e., "It's not my problem")
It is disruptive to other passengers in the cabin if people are regularly moving around and swapping places, particularly if they move their personal effects with them, or if it is a night flight and everyone else is trying to sleep. Therefore I would assert that as a principle of "etiquette" one should take ones seat (particularly an upgrade) with good grace and not seek to play musical chairs during the flight. However, if you only intend to swap once or maybe twice, and you do so discreetly, I cannot see the harm.
You should seek the permission of the purser or senior flight attendant before moving, to avoid inducing confusion among the cabin crew.
You can also ask United "to link" your reservations, so that if one is changed then the airline knows you are travelling together; but this "linking" is not especially robust and as I understand it is effectively just a cross reference in the freeform notes field. However, if there is a need for the flight to be edited it means you are more likely to be kept together.