Você pode usar win7utils para conseguir isso.
win7utils - Windows 7 ISO Disc Image
Utilities
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.
The ei.cfg Removal Utility is a simple
tool that will remove the ei.cfg from
any Windows 7 ISO disc image, thereby
converting the image into a "universal
disc" that will prompt the user to
select an edition during setup. This
tool works by toggling the deletion
bit in the UDF file table, eliminating
the need for unpacking and rebuilding
the ISO, which means that this is
extremely fast (the process of
patching the ISO to remove ei.cfg
takes only a fraction of a second),
and the process is easily reversible
(running the utility on a disc image
patched by this utility will restore
the disc image to its original state).
Alternativamente, você pode fazer isso da maneira manual seguindo estas instruções , que informam como modificar o arquivo ei.cfg para alterar o edição.