The key seemed to be when they accepted the issue and emailed me saying it would take 24-48 hours to locate the account my product key was registered against. After approx. 3 days, I got a further email saying they couldn't locate it and would I send a jpg
of my proof of purchase and product key.
It took a further 36 hours, but I then got an email saying "click here to redeem your purchase" and from then things were simple.
The vast majority of this was done via their online chat, with about 1-2 hours on the phone.
From this, it's obvious they can perform the free upgrade, it would appear to be a lack of knowledge for the support staff as to how to achieve it.
Perhaps pointing whoever you speak to to this thread would help point them in the right direction.
Good luck!