After you have separated your image to a multi-color profile using the GretagMacbeth MultiColor Separation Plug-In for Adobe Photoshop, you can use the MultiColor Proofing Plug-In to convert it to another multi-color profile. Use the following settings:

- Printer Profile: the pre-selected profile (source profile of your image)
- Rendering Intent: Perceptual
- Proofer Profile: your desired destination multi-color profile

Then press the button "Convert to Proofer" and save the resulting image.