Thanks to Marcus in Comments, we learn that some of Onda’s V975i production run has a design that can’t easily upgrade to Windows 8.1.
From a thread at slatedroid:
It is now necessary to solder on two resistors to be able to flash win8.1 bios properly without the HDMI/Screen bug. This is why Onda has delayed the Win8.1 update as they were probably trying to find a way to do so without a hardware modification. Unfortunately, it is pretty much confirmed that you need to solder two minuscule resistors onto an equally tiny spot to be able to flash win8.1. Most of us have really horrible soldering skills. Luckily, if you read on, I’ve come up with an alternate solution that will enable the majority of us to perform this hardware mod with minimal skill.
Like most tablets these days, the Onda V975i isn’t made to be opened by ordinary people. This isn’t an option for people who are fearful of possibly destroying their tablet. For those who don’t want to ship it back to China for a swap, I’d suggest trying to sell it on ebay or Craigslist (and clearly mentioning no Windows upgrade; some people will be fine with just Android) to defray the cost of buying a brand-new V975w. That sucks but this is the risk with Chinese tablets — and especially with Onda, a company that now has a history of fast hardware revisions.