I would not book a ticket on my own unless I was either quite wealthy or my need to be in San Francisco was a matter of life & death. One may, of course, always claim anything one wants. But, collecting is another matter.
A single in J CGN-SFO at walkup prices is likely to be extraordinarily expensive and one takes the chance that EW will not pay and that one has to sue and possibly even lose. In any event, one is out of pocket thousands of Euros for somewhere between a very long time and forever.
Less of a problem if one is on a cash ticket and the cost of the new ticket is not much different. Take the refund on ticket
#1 , but ticket
#2 and then if there is a difference, fighting with the carrier about that amount is less of a strain.