Be careful... W is not always the cheapest upgradeable fare class. I found that route once to be significantly cheaper on a V class than W and is also upgradeable.
I wish united.com would allow you to search "lowest price upgradeable fare".
ExpertFlyer.com can help with that...
(I checked and it seems that W is currently cheapest on your route - $1134 base - and yeah, N is the lowest possible fare with a $278 RT base airfare) GPU = RPU