During college, I trained Olympic TKD. I also trained traditional TKD, palgwe and pyong an forms and teaching both TKD and HKD at the same time.
Because HKD is very eclectic, there are both hard style techniques, such as Tang Soo Do or Taekwondo, but there are also soft techniques. I think the biggest difference is the circularity of movements and utilization of angles as well as body weaknesses such as pressure points and joints. HKD requires at least a working knowledge of many of these weaknesses; however, you will pick up on these as you continue practicing techniques.
I used to "practice" what I learned with friends, family, anyone, that I could get my hands on, literally. One will find that with practice, target areas will become second nature and almost intuitive, but that will come with practice, no shortcuts.
One of my best friends was a Olympic style TKD fighter, and I tried to teach him HKD. I think his biggest challenge was the circularity and complexity of movements. Another friend trained ITF and picked up HKD very quickly so it depends on the person but I think the circularity, angle, and complexity of movement can be challenges.
The biggest challenge one may find during transition, is type of instruction. The Achilles Heel of HKD is the extremely fragmented and decentralized organizational structure. One should research, perform due diligence, understand who the school is associated with and governed by, and the association itself. These factors will play into type of instruction - progressive or traditional. The association that the school is governed by should give insight as to their standards and requirements. Well respected associations will generally have some level of standardized curriculum.
Personally, I am of the traditional camp versus more progressive schools that may draw from other systems. Progress and innovation are good things, but if done out of lack of core content, there is a good risk of getting undesired, unexpected results, whether good or bad, it's not the original HKD that we start out with.
What other styles have you transitioned from to Hapkido? What were some of challenges, or perhaps benefits you had, from transitioning from another martial art?