First off, changing your difficulty to Hard resets your playthrough but leaves your character, pawn, and stored possessions as they are. Changing from Hard difficulty to normal or easy doesn't do that. Naturally, for a soft reset while playing on Hard difficulty you just switch your difficulty back and forth.
Now, I'm on my NG+, and I'm wondering if there are any NG+ exclusive elements in the game such as the Ur-Dragon riftstone in Cassardis that would disappear if the game was to be restarted in the aforementioned manner.
Naturally I lack the cojones to do it myself due to the risk of losing my precious Ur-Dragon access.
Best Answer
I did switch from- and to Hard difficulty and restarted my game. The Ur-Dragon riftstone on the beach of Cassardis still stays, so my guess is that everything that got unlocked by completion of the previous cycle is still unlocked, whatever it might be.
EDIT: Iola (the clothes trader in Cassardis) is still selling her "Farewell" equipment. Safe to say nothing was lost.