I'm still not sure why do you bother checking your installation with Upgrade Assistant - this is neither required nor expected. Results can (and surely are) very confusing.

Windows 8.1 Upgrade is like a Service Pack. if you have Windows 8 up and running, then you should be able to upgrade immediately without any major issues (except some obscure cases - but these are usually very well documented). I can vouch for my Windows 8 installation on the same hardware: it went as smooth as silk once I found the right tile...

In either case your problem seem to be somewhere else: Windows registration process that hasn't got finished / Microsoft Account that is required / etc. I suggest speaking to SONY Support in the first place and - if redirected - following that up with Microsoft.

Good luck!