You can update Products Discounts and Products Specials for multiple products at once, from admin page.
It works with an infinite number of products.

You can filter products by the following conditions:
— Product Name
— Product Tags
— Model
— Categories (multiple selection is possible)
— Manufacturers (multiple selection is possible)
— Filters (multiple selection is possible) (v1.5.5+)
— Price (base price): greater than or equal to a value, less than or equal to a value
— Discount Price: greater than or equal to a value, less than or equal to a value, customer group
— Special Price: greater than or equal to a value, less than or equal to a value, customer group
— Tax Class
— Quantity (stock): greater than or equal to a value, less than or equal to a value
— Minimum Quantity (Force a minimum ordered amount): greater than or equal to a value, less than or equal to a value
— Subtract Stock
— Out Of Stock Status
— Requires Shipping
— Date Available, Date Added, Date Modified: greater than or equal to a date, less than or equal to a date
— Status
— Store
— Attribute
— Attribute Value
— Option
— Option Value

Products for update:
Filtered products will be listed with pagination, so, it works with an infinite number of products.
You can set how many products shown per page (for slower servers or large databases, it is recommended a smaller number of products per page).
You can deselect or reselect certain products from list.

You can set discount prices and special prices as «Flat (fixed) value», «Subtract a value from base price» or «Subtract a percentage from base price».

Products Updating can be done in the following ways:
— Keep old discounts and add the new ones.
— Remove old discounts and add the new ones.
— Keep old specials and add the new ones.
— Remove old specials and add the new ones.

Admin LIVE DEMO (opencart 2.x)
Admin LIVE DEMO (opencart 1.5.x)
username: demo
password: demo
Go to: Catalog > Mass Products Update > Discounts, Specials