So the soul items you have are currently occupying the dual roll of "soul of..." items that don't go away on death and the souls that vanish if you die. Though I haven't said it any where I might as well reveal my intentions, as it may effect the way you play. When you convert souls to XP that is fix, you don't loose it on death, but you also cannot trade with it. When you die, you drop half the souls in your inventory, (where half the souls needs to approximate half the XP value of your inventory as well.) But you will never loose unique souls (anything that drops from a non-respawning enemy), and in general I will be doing half rounds down.
If you can return to the point you last died, you recover any souls you dropped, but as in the original, this only is true for the last time you died, any souls you had dropped 'despawn' if you die before you get there.