Probably this depends very much on the actual student. If a student didn't get fantastic grades because they weren't great at exams they might be absolutely fine on a more demanding course, if their results more reflected their overall ability it might be different.
Temperament comes into it - will a student be thinking "I only scraped in here, maybe I won't be good enough" or will they be thinking "lucky me" and get on with it just fine?
To your more specific question, it really depends on why your DS chose UEA in the first place! If he loved the course and the place then going somewhere else because it's supposedly "better" seems fraught with potential issues. If, on the other hand, he'd always liked the look of a course t Nottingham but assumed he couldn't get in then why not have a second look?
I'll admit I'm always puzzled by the way a few Mumsnet posters assume employers are au fait with the exact league table positions of unis. Sure there's a premium for Oxbridge over Wolverhampton but I'd very much doubt if your average local government grad interviewer would know any difference between say Newcastle, Reading, UEA. Southampton.... when I was applying back in the 80s all these would have been solid choices.
On a personal note, DD is at a mid ranker (Aberystwyth) and I think it's suited her temperament not to be worrying that everyone else might have better grades than her The uni have also allowed her to drop from joint to single honours for the last year because illness means she can't do a year abroad. We were very relieved they weren't going to make her resit a year. Maybe a higher ranker would have been less helpful!