UA overbooks C all the time--that's nothing new.

Next to HI, SYD is probably the next most popular "dream route" on which awards and upgrades are used. And for good reason--7,400 miles is no short jump.

Similarly, people who make this trip--especially the "once in a lifetime" travelers from the States--will even pay for C or F to Oz.

Also keep in mind that March is a great time to be in Australia since the end-of-summer is still warm, but in a few months they have winter.

I wouldn't say your chances of upgrading are hopeless--there are 73 (probably more) people booked in C--if you're throwing a party and 73 people are supposed to come, usually only 65-67 show up. So, it'll be close for you--fare class will probably play a role--maybe one of the resident UA geeks will do some checking for you?