I've done this is the past - as long as there is upgrade space showing as available there is no problem with using miles to get a confirmed upgrade. As others have noted, UA's holding back of the NC and NF inventory is the problem.

Upgraded on UA several times on Trans-Pacs. I became 3P in July and 2P in October 2008.

Jan 2007 - SFO-LAX-NRT and KIX-SFO: business class, open jaw
Sept 2007: SFO-NRT: business class, one way
March 2008: NRT-SFO-NRT: business class
August 2008: NRT-SFO: business class, one way

Not sure what the chances are now of getting those upgrades, but they weren't impossible.

I believe that it used to be possible to upgrade high fare bucket tickets on other *A carriers with UA miles, but not anymore.

But zcat18 you do not seem to understand that upgrading with miles has little (for your purposes) to do with status. If NC or NF space is available, and you've puchased a ticket in a qualifying fare bucket, then upgrades are confirmed immediately regardless of status. Status only comes into play if you are waitlisted.

Re E- vs E+, E+ is not a higher class of service thus one cannot "upgrade" to it. While E+ seats are sold to GMs at a premium it is not by definition an upgrade.

Most of the answers seem UA-centric, so here's a view from another airline.

A little over a year ago, I wound up having to fly on American trans-Atlantic. I'm not an elite with AA (yet -- should make Million Miler soon, mostly from non-flight miles, and that will make me Lifetime Gold), but on the day of departure I asked at LHR whether I could upgrade using miles, and was told "sure". Needless to say, I did so.

_____________________
MM=> UA 100.7%, AA 99%

gre- Thanks for the explanation!

Counsellor- Sometimes I wonder if I should switch to AA as my airline of choice. All miles programs are equally frustrating, but sometimes I wonder if AA makes things just a bit easier than United does. I've just had consistently bad luck with AA from a service and on-time performance perspective, so I became a United guy by default (plus, *A is the best alliance in terms of global coverage, IMHO).

Yes, United seems to hold potential upgrade seats until the last minute, which can get very frustrating. However, AA did charge me a $100 fee (I think it was) for processing the upgrade since it wan't requested something like two weeks before departure. Last I heard, United has no such fee (shhh!) and allows last-minute upgrades. Just last week I had to fly trans-Atlantic on one day's notice, bought the ticket (a "V" class ticket) on United, and upgraded using an SWU. Went right through, no muss, no fuss -- and no "late booking" fee.

As always, YMMV.

UA miles on LH TATL
 

If flying on a H/M fare I've used UA miles certs to upgrade from LH Y to C frequently, although rarely does it clear in advance, but I'm 5/5 over the past 1.5 years in both directions at the airport (remember it's only one segment that upgrades, any connections stay where you bought them, and you need to have the CERT in hand...)

To quote A Tribe Called Quest, "before I had status and before I had pager..." I requested an upgrade from C to F on a LAX-SYD flight, and got it.

It helped that it was a refundable fare...in fact, it was that legendary mistake fare, the r/t LAX-New Zealand for $1300.

Funny, that trip got me status on UA...and I haven't gotten an upgrade since!

I sat next to a woman on UA's Friday evening IAD-SEA flight. She had no status with UA, but had managed to upgrade at the kiosk using miles. I was pretty surprised, as I assume this is a fairly tough upgrade.

The strange part is that while a non-status pax was able to upgrade to F using miles, I (as a 1K) did not get my meal choice! Apparently they only had been catered to 13 of the chicken entrees (for a 24 F 757). I questioned whether there were actually 13 other 1K's / GS's in the cabin, and she replied, 'everyone here has 3 stars tonight', which obviously wasn't true considering that I was sitting next to a GM!

5000 miles coming your way :) :D

Huh? Was I do a skykit?

This is the first time I've never received my meal choice, and while I did question the FA to see if procedures had been followed (since she was not very apologetic about it) I just ate the rubbery pasta and went on.