In my experience, if there is an equipment change from a 747 to a 777 then your upgraded will be 'protected', assuming of course there are seats available. Typically you will automatically be given a new set assignment, in rows 8 or 9 (on a 777). You can of course ring up UA and get a better seat assignment.

Are you worried about a particular segment? On a particluar day? SFO/ORD-HKG should stay as a 747 and HKG-SIN would definitely stay as a 747 (assuming of course these flights remain part of UA's regular service).

SIN-NRT-SIN does change from a 747 to a 777 right at the end of March, 2003.

Some SFO-NRT flights change from 747's to 777's and vice-versa, throughout the year.

In the event that you are downgraded (back) to Economy for one segment, you'll have to decide if it is worth the miles for upgrading say, just one segment.

I believe that all current reservations, through 31 March 2003, have been modified to account for equipment changes so once "My Itineraries" responds a little more reliably for you, you should be able to view any equipment changes.

I assume that, in the event UA drops service between SIN and/or NRT & HKG, that you would be accomodated on another Star Alliance Carrier, like ANA or Singapore Airlines. UA has accomodated me on JAL, for NRT-BKK, but more as a result of a missed connection due to a late departure.