Just a little more information about Master Hyun's teacher:
Kim Jung Yoon (Kim Jeong-Yun) was one of Master Choi Yong Sool's most senior direct students in the 1960s. He was appointed by Master Choi to one of the most important positions in the first hapkido organization, the Daehan Kidi Hoe (Korea Kido Association - presently run by Kuk Sool practitioners), over people like Master Ji Han Jae, leading Ji to form his own organization the KHA.
Having seen some of the many books he published in Korea on his art of Han Pul (Korean technique) I visited his dojang and interviewed him 1996.
He spoke a great deal of Choi Yong Sool and no other teacher while I was there. He did talk about historical research into the background of traditional Korean arts but most of what he was talking about was academic research into old texts (which is where he came across the term 'hanpul') and he never mentioned studying with any other teachers specifically.
When hapkido started it was called Yu kwon sool, then Hapkido, then by the early 1960s Kido, then hapkido again. He published some of the earliest hapkido books in Korea. For Master Kim to have used another name for the art without changing the content of the art much would not have been a very big deal.
Master Hyun has always referred to his art as hapkido, I believe. He may have came to the US before Master Kim started employing the term hanpul.
It was my understanding that Master Hyun no longer does much of the teaching at the school himself (This may be incorrect. It was just what I heard.) and rather has hired younger hapkido teachers who graduated from Yong-In university, who majored in the art.
http://www.hapkido.ws/staff/grandmaster/