you just turn off the notifications in the stock messaging app. then use the one you downloaded. i dont know if thats what you mean by "turn off," but thats how do it

 

+1

You can't "turn it off" to my knowledge, only turn off notifications with the Android messaging app. This is what I do since I run Handcent.

When you're in the stock messaging app:
Menu Key -> Settings -> Scroll down to "Notifications" and uncheck.

 

go into contacts and swipe to the left for a message. It'll ask you which you would like to use as default...select ur new client =) Took me a LONG time to figure that out. I was getting two sets of texts.