Update thumbnail sizes

Ciao,
I have noted that, after having already added some thumbnails to my posts, if I change thumbnail sizes in Settings » Media (or I add new thumbnail sizes in functions.php) the new files for the new thumbnails sizes are not created (crunched). Actually it seems that the only solution to update thumbnail files, crunching the new ones, is to delete the image from the media and to reload it… a long procedure if you have already uploaded a lot of files.
What about adding:
1. an “Update all thumbnail sizes (re-crunch)” in Media Library;
2. an “Update thumbnail sizes (re-crunch)”:
2.1. to group actions selector in Media Library,
2.2. near to “Update media” button in Media » Edit Media and
2.3. near to “Update image” button in Edit Post » Gallery
?
Thank you very much