Can you swap them with other people who have days they also don't want?
Technically not, but last year, some friends of ours couldn't make it to london because of the floods. So they posted their show court tickets for the Wednesday by recorded delivery and they arrived at our hotel at 10.30am on the day and we had an extra days play to see.
Was quite fabulous. No one checks for names or anything.... not sure they even are specific to people tbh.