Other than a slight speech delay in the early years, there's no difference between AS and HFA - they're the same thing. The only reason I can see for switching a diagnosis from one to the other is the buzz that the term Asperger Syndrome may be dropped in favor of calling both HFA when the next version of the Diagnostic Manual is published. That's all just speculation at this point, but in the end as far as their effects on a person's daily life, the terms are interchangable.
I find its a little easier for most people to grasp if I tell them I have a form of High Functioning Autism, than if I use the term Asperger Syndrome. At least they've heard of Autism and recognize it as some sort of disability. Since most people pronounce Asperger Syndrome as Ass Burgers, they just want to snicker at it rather than take it seriously (the man's name was actually pronounced Ahz-pair-gur).