Their website has now been updated. It now says on intercity services (which I presume means Edinburgh/Glasgow-Aberdeen/Inverness) you can now upgrade for £10 at weekends and £15 weekdays, and on Edinburgh-Glasgow services it's £5 at weekends and £6 on weekdays.
I didn't know ScotRail also offered upgrades on weekdays. Is this new, or have they been doing it for a while? Presumably on weekdays they are only available on off-peak trains.