Need to change the sign for an iPhone app

[GregLBS]

I have received an iPhone app myapp.app from a developer. I also was given the following bin file (see below) that supposedly changes the signer of the app to my sign. So I can now distribute it to 100 user's using my dev account. I run the script it returns the following "myapp.app: replacing existing signature" which I believe is correct.

However when I drag the app to iTunes and try and install to an iPhone we get the message "The app "myapp" was not installed on the iPhone "My iphone name" because the signer is not valid"

First question is do people think this approach and script should work?

If yes any ideas why it doesn't?

script...
#! /bin/bash
export CODESIGN_ALLOCATE=/Developer/Platforms/iPhoneOS.platform\
/Developer/usr/bin/codesign_allocate

codesign -f -s "iPhone Developer" myapp.app