See this from UA's website: Passengers requiring extra space, United Policy

I agree that UA should be following its own policies in a situation like this. However, has anyone actually observed UA trying to charge a passenger in this situation extra or denying him/her boarding?

Also, has anyone successfully requested a seat change from a FA on a full flight from this reason? I'd imagine you'd just get an eye roll and a comment that you should have bought C or F class if you wanted your space.