Hello Quaizexi59
Welcome to the SBB Community.
Although we do not recommend purchasing tickets from third-party resellers when the sane ticket is available from SBB, saver tickets cannot be exchanged due to change of travel plans, and this no matter the point-of-sales.
Saver Day Passes are neither exchangeable nor can they be cancelled.
Whether SBB makes an exception for 8 May since you intend to purchase a Swiss Travel Pass? You have to ask SBB. We are a travel forum, not the SBB Customer Service: +41 51 225 78 56 (24/7, waiting times may be possible).
Kind regards,
Roland