If you are:
1) an MVP, and
2) you buy a Y/S/B fare, and
3) there is U space at the time of purchase, then
You will be upgraded immediately. You must be the only person on your PNR to receive the complimentary upgrade.
If you are:
1) an MVP Gold or MVP Gold 75K, and
2) you buy a Y/S/B/M/H (Value) fare, and
3) there is U space at the time of purchase, then
You will be upgraded immediately. You must have no more than 2 people on your PNR to receive the complimentary upgrade.
If there is not U space at the time of your purchase but you still meet criteria 1) and 2) for your elite status level, you go on the upgrade waitlist immediately and are eligible for any U space that becomes available (after those already on and above you on the waitlist have cleared of course).
You can always identify whether there is U space available by shopping for the appropriate upgrade fare: in the upgrade preference dropdown, select "MVP Upgrade" or "MVP Gold Upgrade" depending on your status level. A gold U will show up next to those flights that are upgrade eligible and have U space. If you see a U with a slash through it, it means that there is no U space available, but you can still buy the expensive fare to waitlist immediately.