Hello Tetel
In this case you have to cancel your Swiss Travel Pass and purchase a new one for a day earlier. Or, you use a Saver Day Pass instead:
https://www.sbb.ch/en/tickets-offers/tickets/day-passes/saver-day-pass.html
If you want to cancel it, contact the point of sales. If it was SBB, then you can request a refund here, a CHF 10 admin fee will be deducted, though:
https://www.sbb.ch/en/help-and-contact/refunds-compensation/refunds-compensation/refund-form.html
No, if you use a ticket whose reduction is based on the Swiss Travel Pass, the Swiss Travel Pass has to be valid, too. The legimation for a reduction is being checked on the day of actual use, not on the day of purchase.
Kind regards,
Roland