Hello Whooshiha17
Yes, an upgrade of your Swiss Travel Pass to 1st class is anytime possible. You can purchase a point-to-point or a day pass upgrade, whatever is more cost-effective:
https://www.sbb.ch/en/tickets-offers/tickets/point-to-point-tickets-switzerland/class-upgrades-supersaver-class-upgrades/class-upgrade.html
Alternatively, you could contact RhB rail service, which also forms the Glacier Express customer service team. There might be seats lefts in 2nd class which aren’t visible in the reservation tool as they were blocked for groups. You may check with the customer service team firs; from my personal experience, they are very helpful and do speak English:
E-Mail info@glacierexpress.ch
Phone: +41 81 288 65 65
Please let us know if you need further help.
Best regards,
Roland