There is no word about S fare upgradability on SWU-2001.
S is NOT neither in upgradable fares list nor nonupgradeable fares list.
In 2001, I was denied upgrading S to C with SWU-2001 by UA Japan office.
A large number of 1Ks in Japan reported the same experience to Japanese local BBS.
UA-Japan office always gives restriction that never heard in other countries to member's benefits.
So I sent e-mail to customer service twice to ask if S is upgradable or not, but both reply only said "Ask office in your country".
I have no idea whether it is one of the UA-Japan office's "original" restrictions or UA-HQ's.
But at least in Japan, S fare is NOT upgradable even by SWU-2001.