Can't use the e-cert for Japan though.
This one might be a bit unclear, but it seems you can't upgrade to WBC with this even if you buy a "b" fare with it.

Upon completion of your qualifying trip**, you will automatically receive a Dollars-off E-Cert via e-mail. E-Cert is valid for up to $400 off* a qualifying paid roundtrip to Beijing, Guangzhou or Shanghai, China; Seoul; Hong Kong; Singapore or Taipei.

This E-Cert offer may not be combined with any other discount, promo fare also known as a web fare, infant/child fare, senior fare, certificate, E-Cert, ticket designator/reference code, bonus, upgrade (except WorldPerks Elite card and Elite companion upgrades), WorldPerks Air Travel Award or group fares