My name is Alison, and I work for Ablestar apps. Our Bulk Editor app is capable of bulk editing product prices and compare at prices in bulk. You have the choice of using an in-app edit, or uploading a spreadsheet with the new prices in it. This document will walk you through an in-app edit that decreases the product prices by 10%, https://support.ablestar.co/article/27-your-first-bulk-product-edit.
In addition to bulk editing product fields, the Bulk Editor app also allows you to 'undo' edits, and depending on the plan you are on, you can schedule edits as well. There is also a free plan available that allows for 5 free edits per month, so you can try the app out without incurring any costs.
If you have any questions please feel free to reach out to us at firstname.lastname@example.org.
This can be easily done with Airtable - you can set prices in bulk to a specific number, or use a formula to set based on a percent increase/decrease or any other logic you want (e.g., increase price by 30%). Check out our overview of how to bulk change Shopify product prices with Airtable
Is the 'compare at price' higher than the 'price' for the products you need to edit? They will need to be higher for the prices to show as on sale.
The format of the sale, and the way it displays is controlled by the theme you are using, but generally all Shopify themes follow the same rules, the 'compare at price' needs to be higher than the 'price' for the product to show as on sale.