I tried to convert the AP "Serpents Skull" from Pathfinder to HARP.
Its a lot of work with all the enemies, monsters and traps. PF is a very combat-intense rpg, HARP is a lot more lethal. So every fight might end in a tpk. Chances are way higher in HARP, than PF.
The conversion of the encounters is not done lighthearty. Some Monsters can be taken from the HARPO Bestiary, but ... BUT ! at least 80% of the encounters must be adjusted very carefully. They have special powers, spells, magic items. To balance that out, it took me so much work, that we re-changed the system back to PF. Sadly.
And to run an AP is a lot of work for the DM. You must be prepared for everything. My players love to crush the plot within minutes, find solutions no designer thought of and do terrible things I never expected and prepared.
So if you prepare an AP and change the rules-set, you have a lot of homework to do.
(please apologize my bad english)