Baggage allowance is based on your purchased ticket class, not upgraded class.

http://www.united.com/page/article/0,6722,51146,00.html

You can ask check-in agent for a box. Transfer 12 lbs to a box and problem solved.

My sister-in-law and brother had to pay a hefty penalty for checking in two 70 lbs bags each person from the U.S. to Asia. They thought that was their limit since they upgraded from Y to C.