I received the following response from HK support:
Thank you for the inquiry. If you are looking for an firmware upgrade for your AVR635, currently is unavailable as the website is being restructured. You may want to check the website for the upgrade after couple of weeks. I apologize for the inconvenience.
I hope this helps.
HCG Customer Support"
That said, hopefully someone can come through with the update via email as it doesn't look like HK is being too cooperative.