Can I upgrade from 10.8 to 10.9 with a bootable usb installer? I've got a working bootable mavericks thumb drive, but I haven't the installer from Mac App store...

Can I upgrade from mountain lion to mavericks doing the installation from usb drive or  must I do a clean install?

Can I boot from usb drive and upgrade, or when I boot from usb I'm forced to format and do a clean installation?
Yes, simply run the installer from inside your existing OSX install.