If I was you I would pop down to your local breakers and have a good look at the location of the CECM whilst down there cut off one of the three large connectors and open it up at your leisure at home, it's simple enough but one persons simple is another's hard!
If your upgrading your CECM to a 30 byte unit I would definitely go down to the breakers and do a practice run on their connectors and practice how to release the red bars on the CECM and photograph the layout so your comfortable with your pending mod.
It's really very simple task but easy to break plastic when you've never done that mod before.