Originally Posted by findark

Your ticket has a fare from SFO to BCN, not two fares SFO-EWR and EWR-BCN. Therefore, it's relatively meaningless to talk about "the cost of only up-faring one leg". To change only EWR-BCN to W would require inserting a fare break in EWR, which would almost certainly be more expensive than changing the entire ticket (with normal combinability rules, to keep this all on the same ticket, you would probably end up with EWR-BCN in B or full Y).

While UA has some nebulous "SDC to a higher fare class" concept, I've never understood where they derive prices. Regardless, it is almost certainly the most economical option to simply change the entire ticket to W.