Compare the actual food ingredients in Hill's and I suspect you won't want to change foods. You can always add the supplements yourelf. Hyaluronic acid is very good for the knees.
Cartrophen is not available in the US, only Canada, UK, AU, etc. It's similar to adequan, but supposed to be better. Wonder if an adequan injection would help. Of course, omega-3 fatty acids, cetyl myristoleate, DLPA, may all help with the pain.
Can't see the ingredients of that product on my phone, but homeopathics like arnica, traumeel and zeel are often useful in joint arthritis conditions, this one may or may not help.