Can the new DELL XPS 12 ultra portable tablet's hard drive be upgrade din the future by a customer?
There are screws and the bottom is accessible. So theoretically you could install a new larger hard drive, however that would void the warranty that the machine comes with.
My best suggestion (to save some money also, b/c we know large internal SSDs are very expensive) would be to buy an external drive to use with the computer. There are even pocket sized small portable external drives you can buy inexpensively.
Hope that helps,