Hi Ingwe,
When you reroll a value that's already existing, it has a chance to fall anywhere in the range of 1-X. X being different for every stat. Your Harm example is accurate. Another example is on the Chiv's Shades, the Damage Increase is +5. Because damage increase per item is around 20 or 25, there's a good possibility that spellcrafting that stat will result in an increase, but hey.. it could drop it to +1 or destroy the item.
A negative, such as the one you've mentioned should in theory spellcraft to a positive as spellcrafting can't result in a negative value. I have not tested that specific item. Another test I've failed to really vet out is trying to spellcraft +durability onto the glass sword. It's always blown up in my face.