Is it possible to force iphone/ipod to update apple-touch-icon once webapp is added to home screen?
It's been a few years since this question was asked/answered and I'm here to report that this is now possible!
In newer iterations of iOS, the Apple Touch Icon that is displayed on the homescreen is cached just like any other piece of content from the website. By simply changing the name of the image, it will force the homescreen shortcut to refresh the icon the next time the shortcut is launched.
However, to keep in line with Apple's naming conventions, you can simply append URL variables to the reference whenever you want the icon to be redownloaded and continue to call the image apple-touch-icon.png.
What I have done to automate this issue (in PHP) is append the last modified date/time to the image. For example:
<link rel="apple-touch-icon" href="apple-touch-icon.png?m=<?php echo filemtime('apple-touch-icon.png'); ?>" />
This outputs:
<link rel="apple-touch-icon" href="apple-touch-icon.png?m=1415832495" />
Now, you don't have to do anything but literally overwrite the image and everything else will happen automatically. When you change the image, the modified time changes and doesn't match what the user has in cache anymore, so it forces a new download. Simple!
OLD ANSWER: I've done a ton of research into this trying to find a way. Unfortunately, it is not possible. It needs to be removed and re-added to the home screen for the new icon to be used.
I just tested a theory on the Iphone 5 and Ipad 2, and it worked for both. Similar to the favicon, you can force the new favicon to pull by changing the href.
Instead of href="/apple-touch-icon.png" I used href="/apple-touch-icon.png?ver=1.1" and it pulled the new icon when the site was visited (without deleting and re-adding to home screen).