Resizing images and fixing posts/pages

Hi,

I would like to have all my images resized. I have read http://wordpress.org/support/topic/cant-change-image-size-via-settingsmedia?replies=5. What I do need is the option to also have the image sizes adjusted in the posts that I have written so far, i.e., e.g. if I change from 320×240 to 640×480, I would like to have the post

<a href="http://XXX/wp-content/blogs.dir/1/files/2010/12/20101225-1005191.jpg"><img class="alignnone size-medium wp-image-1503" title="20101225-100519.jpg" src="http://XXX/wp-content/blogs.dir/1/files/2010/12/20101225-1005191-320x240.jpg" alt="" width="320" height="240" /></a>

changes to
<a href="http:/XXXX/wp-content/blogs.dir/1/files/2010/12/20101225-1005191.jpg"><img class="alignnone size-medium wp-image-1503" title="20101225-100519.jpg" src="http://XXX/wp-content/blogs.dir/1/files/2010/12/20101225-1005191-640x480.jpg" alt="" width="640" height="480" /></a>

Is there a plugin that does this for me?

Thanks a lot,
Mercator