Originally Posted by jlarroulet

LAN does not offer paid upgrades. You can only use your loyalty coupons to get listed for an upgrade on standby (in the T-48hr window, after checking in) or, if you are Premium Silver or Comodoro you can use your kilometers to get a confirmed upgrade as much as 6 months prior to the flight (you'll have to deal with LAN's contact center though)

If the Y trip you purchased allows a full refund (depending on your fare family) the easiest way would be to request the refund and purchase a J fare.
Keep in mind that LAN sometimes offers a J fare that's very close or even lower than available Y fares because those are non-refundable, non-changeable, non-nothing