https://wordpress.org/plugins/imgoptimizer-png-to-jpg/

ImgOptimizer & PNG to JPG helps you reduce Media Library image sizes without changing image attachment IDs.

• Process JPEG and PNG images in batches.
• Resize images that exceed configured dimensions.
• Recompress JPEG images at a configurable quality.
• Recompress PNG images while preserving transparency.
• Convert PNGs to JPEG only when no transparency is detected.
• Process a single image or selected Media Library images.
• Optionally process new image uploads.
• Roll back any processed image to its original file, individually or in bulk.

PNG-to-JPG conversion changes the image URL extension. Keep the original PNG until you have checked affected pages, especially when a page builder stores image URLs in custom data.