Hello Doiniroo86
Good day.
Working as a ticket inspector, I have never seen a Luzern-Interlaken Express day pass. And, you probably mean a Swiss Travel Pass, not a SwissPass.
I assume you have got a Saver Day Pass for the Luzern-Interlaken Express. These are neither exchangeable nor cancellable.
Please contact SBB (phone: +41 51 225 78 56) to see if they do an exception. Normally, you must first purchase a Swiss Travel Pass before an exception is made.
KInd regards,
Roland