1. What is Scr.im?
You may read more about Scr.im on its about page.
2. How do I change the Email addresses for the posts that have been already posted?
Well, there is really no method, but you may click the "edit" link below the post which has emails, and then click update post. The plugin would then convert all Emails in the post to Scr.im URLs.
3. If I disable the plugin, will all the scr.im URLs vanish?
No.
You may also read some questions on the scr.im's FAQ page.