You nailed it, Jean-Luc. But that FAQ isn't clear about $10 upgrade. It only mention clearly, the minimum pledge $20 as the starting point. So probably there isn't such an option. Maybe after Kickstarter campaign.
I'm backing other projects in Kickstarter, if one doesn't achieve the minimum funding, I'll pledge the extra money in Xenonauts. So many interesting projects in Kickstarter to backup, that I'm close to bankrupt.