I would hope that your internal processes such as passing the upgrade and being supervised by a team/second supervisor would prevent this happening, it is fairly rare these days at our uni except where the candidate just doesn't have enough to give and really is at a MPhil standard.
I second the advice about getting the right examiners though. You need someone who is firmly within your discipline or firmly within your topic, and ideally both. It's awful to end up with lots of amendments as you fall between disciplines, this happened to me to some extent.
The other way to look at a viva is that it is just a chance to find out what amendments you really need to make! If you think of it as the end point pass/fail, this can be disappointing if you get amendments. I had a PhF student though, who just couldn't quite seem to finish off his PhD and in the end submitted it though I felt it could have been improved by about 10%. He did get major amendments but at least he knew once he'd done that extra work, that was the end of it so to his mind, this was a victory of sorts.
If you can find someone to read your thesis and look for major flaws, great. I wouldn't worry so much about style (unless it's unreadable) or tinkering with minor things at this stage, as unfortunately examining is quite idiosyncratic and you may be changing things that your examiners don't care about.
Have you already chosen the examiners and are you happy with them?