If you want to use your old case, dc3 is totally correct; I have reused my Apevia X-Dreamer three times also.
I am trying to remember some of the problems I have had when replacing new parts on an old case, some things come to mind:
> Your old case probably does not have front USB3 ports. USB2 will probably be gone within a couple of years.
> Make sure that any new mainboard features that are supposed to hookup to the front panel of the old case will have proper leads and sockets to receive them.
> If you are going to want to game, study the ventilation setup of your old HP case, and the options of added case fans. What if you want to add liquid cooling down the road? Will there be clearance? What if you want a 120mm fan but there are no 120mm openings & mountings?
>Make sure the power supply mountings & screw holes will match your case, and if the PS you want to use will even fit in the mounting area.
The problem with these retail cases is that they were engineered strictly for the mass-produced custom made components that were to go in to them; sometimes the newer components don't fit and you must mod or be imaginative in installing. HP, Dell and Acer don't care about upgrading, just selling what they have at the lowest construction cost.