In a recent theme update, the width of the theme's main column was widened from 690px to 770px, and the overall site width was widened from 1050px to 1200px.
These changes mean that all featured images previously uploaded as full width or at main column width are now too small.
What is the best method to efficiently and effectively resize the legacy images?
In an ideal world, this could be done systematically with some kind of review process for images that get too blurry. Images that scale up okay would be left alone, ones that are too blurry could be replaced.
Note that not all of the images were sized precisely at the theme's legacy max width dimensions. Some 1200px images were uploaded with the theme reducing them to 1050px. These should be fine as is. Same for images that were 800px or more put in the 690px spot. However, of the hundreds of featured images in the media library, it's unknown how many images are 770/1200 or more and how many didn't meet those dimensions. So any systematic resizing would obviously ignore the ones that meet the new dimensions.