For op-upgrades, dressing up can help more than it can hurt.

However, it can hurt:

Back in college I took part in a learning group that met at the USC. I often flew home on the 2:00 pm 744, LAX-ORD (R.I.P. ). This was back when domestic biz had printed menus & hot towels. Ahhh, those were the days.

Anyway, as a college student, I often just dressed comfortably, and during the summer that meant a pair of decent cargo shorts & a polo. (This was also pre-luv handles, when I actually RAN marathons, not exhausted my way to the finish line, and I could eat anything I wanted without the visual effects. Ahhh, those were the days. )

I was waiting at the gate for an u/g on UA104, as a 2P with 500 milers. I approached the bench before they had called ANYBODY, and asked the agent how my chances looked. She was younger, about my age, and cute...after a quick lookup and a bit of flirting she said, "hmm, your chances don't look good, but let me try something." I left there with a biz BP, 15A, and I saw numerous people lingering around who did not clear--certainly one (or more) of them had to be 1P or higher.

In this instance, I think the GA "fancied" me as my appearance appealed to her style, so she was probably happier to give someone of her age group an upgrade rather than the stereotypical businessman.

I've also noticed on various flights that being younger and casually dressed can get you better service from the younger FAs (again, you're on the same 'level' as them--so to speak). But now, with the furloughs and layoffs, there haven't been many younger UA FA's...so it's best these days to dress a little nicer for the matrons!

And finally, if your u/g is confirmed, dress comfortably!!! No need to impress in a plane, IMHO.