Category sales, like the old site's Pricing Rules. Turn a rule on and every product in its collections gets the sale price, with the normal price shown crossed out. Turn it off and those prices go back. One product only ever gets one rule (the highest %). If a product already has its own sale price, it gets whichever is lower: its own sale price or the rule price, and turning the rule off puts its own sale price back. Trade members still get whichever is lower: their trade price or the sale price.
Just type the new normal price into Price on the product page and save. Within a few seconds the sale % is put back on top of the new price automatically (Price becomes the sale price, Compare-at becomes the new normal price). Works for price rises and price drops. To give one product its own special price instead, open the variant and change both Price (special price) and Compare-at (normal price); the customer then gets whichever is lower.