1) The GUID is exactly that — a GUID. It’s used to uniquely identify the post. If you need to link to a post, then use get_permalink( $post_ID ) ($post_ID is optional) (link: get_permalink).
2) Not without a plugin, no. There’s talk of using an image shortcode for 3.1 though, or maybe 3.2.
In the meantime, you can try using an alpha version of my Regenerate Thumbnails plugin:
http://viper007bond.pastebin.com/XprbYtg2
It’ll go through all of your posts and update all image tags.
Make sure you back up your database first though. The code is alpha and not guaranteed to work, although I have tested it quite a bit.