I believe most are overreacting, as a 1K prior to conversion I was not upgraded out of SEA and every seat was filled. The OP's offer most likely was a buyup to the next instant upgrade fare of Y/B/M. If you want to know your chances take a look at how many seats remain unsold in Y, if it's nearly full and your on an S fare forget about the upgrade.

Besides UA got the original UDU for elites in 2010 because of the alliance with Continental AFAIK.