Not sure about this, but it seems like a negative penalty on a ring of leeching doesn't change to a positive ring of leeching even after enchanting it to a positive bonus.
Viewing post in Bug Reports
So, the scenario is, you have a -1 ring of damage, use an enchantment scroll without unequipping the ring, and the bad effect is still there? Yeah, that is a bug I will fix next patch. Thanks for reporting!
Un equipping and equipping the ring again will solve the problem, but I will work to solve this bug to avoid that