The is a simple tool that will remove the ei.cfg from any Windows ISO disc image, thereby converting the image into a "universal disc" that will prompt the user to select an edition during setup.

The Windows 7 ISO Image Edition Switcher is a set of small binary patches (and a tool to apply these patches) that will convert an official Windows 7 ISO disc image into an official Windows 7 ISO disc image of another edition. The resulting ISO images are bit-for-bit identical with those posted on MSDN or TechNet, and their SHA-1 hashes should match the official hashes posted by Microsoft.

Download here - ei.cfg Removal Utility.