The Content Builder is designed so that you can only edit content that you've created with the Content Builder plugin. It's not possible to edit content that you've already created using the standard WordPress editor.
Why Can't I Edit Existing Content?
The WordPress content editor uses something called TinyMCE. This editor is continually being updated (recently WordPress included a new version of the editor in version 3.9) and the way the editor behaves is constantly evolving.
Therefore, to avoid conflicts between the WordPress editor and our own editor we wanted to make sure that both can be run alongside each other.
Therefore, as a user, you can decide which editor you'd like to use for each piece of content you create giving you the maximum control.
Can I use both the WordPress Editor and Content Builder on the Same Page?
Yes you can - the Content Builder content will always appear above the main WordPress editor content. There is nothing to stop you using both on the same page, however.
You can also use the Content Builder on some of your content and the main WordPress editor for other pieces of content.
Should I Migrate my Existing Content across to The Content Builder?
Unless you're going to completely redo all the content that you've created on your site then there's no advantage to doing this.
We recommend that you simply leave existing content in the WordPress editor and use the Content Builder plugin to build your content moving forward.