Hey @mtrexler
There is information on changing the build image in this post and in this documentation. It will look like this
You would need to go into the settings for each site and see what build image is used, then change it as required.
Only sites that are connected to a git repository will have these settings and require changing. Sites deployed manually will not have a build image.
Another option is to use the Netlify API listSites which includes a build_image key.