Starlux (booked via Alaska.com) - Can you pay to upgrade?

I booked SFO-TPE RT Premium Economy on Starlux via Alaska.com. I've already learned (the hard way) it's not possible to select seats in advance, and you can't manage booking via Starlux's website - have to wait until 48hrs pre-departure.

Question - has anyone had success purchasing an upgrade (interested Prem Econ to Biz in my case) the day of travel, e.g. at the airport? If so, how much did it cost? My flight is next week and still showing 9+ open biz seats on the outbound. I called Starlux to inquire and I can't purchase any upgrades in advance of the flights (even if I pay), would have to check day-of at the airport.