Old images not deleted!

Changing the size of existing formats is not advisable – all your old posts with embedded images have hard-coded URL:s. (For example image-300×200.jpg) If you delete the old thumbnails from the server, you will have broken images in your posts.

If you still want to remove the old images after the advise above, you can use the Force Regenerate Thumbnails plugin:

https://www.ads-software.com/plugins/force-regenerate-thumbnails/

https://www.ads-software.com/support/topic/does-deleting-size-delete-previously-generated-images is a duplicate request for this feature. My latest post there contains links to other threads requesting the same feature. It would be a very good addition. I haven’t tried the plugin Khromov linked to, and it could work, but it’s last been updated in 2012 and has some reported issues with non-English-alphabet-characters in filenames.

Having the option in Simple Image Sizes to delete old thumbnails would be very helpful to many.