I think on the website, it says that you cant with advance
Whilst you're right MCR247, VT say that you definately can upgrade an advance SC single to First Class using the £15 weekend first promotion (subject to the usual condition of space being available etc).
It's just Cross Country being very tight (as per normal) with tickets etc, who say that standard advance ticket holders can't upgrade- Neither do they allow standard anytime ticket holders to upgrade for free on XC services- regardless of the fact anytime ticket holders have almost certainly paid well over the odds for their ticket in the first place...