HI Brian,
If you are using 9.1 there is a bug that will zero out all discounts if you try to add a new one or update one single row.
The best and safest option is to extract all data into excel, add or update the detail you need to change, then re-import everything.
Very time consuming if you have a large database I know!
Let me know if you are using a lower version & I will give you the detail