Yes, that would void the warranty. These tablets are not meant to be opened unless there is an issue. Then only a service center would be permitted to open the tablet.
I would careful about which site you purchase from. I am currently having issues with Gearbest as my keyboard arrived smashed. I paid for the insurance and expedited shipping. They don’t want to refund me for the extra shipping cost. It is only a few dollars but that’s not the point. I shouldn’t be out anything.
It’s a shame it actually isn’t allowed to open it for upgrade purposes. Which way they can see whether a tablet has been opened or not?
I haven’t decided yet where I’d potentially buy one, I saw there’s also a Cube Mix Plus with better specs but there seems to be quite a difference in price.