Bring in the model with the diffuse, and use transfer maps to project the diffuse onto the new (normal map) UV layout. Or use 3dsmax's Render To Texture and projection to do the same thing. As long as the models share the same space it's a snap. EDIT: Watch out for Global Super Sampling and anti alias settings they can…
It would probably be easier to transfer the diffuse to the new UVs than to change anything with the normals if they're already looking the way you want. If Maya doesn't have a tool that lets you transfer information and textures between different models, you should be able to use a tool like xnormal. Objs should even work…
Alright so, I'm working on a character and made kind of a stupid mistake....I have a version in Max that is setup for diffuse, and a version in Maya that is setup for a normal map. Each version has a separate UV set layout and the normal map is already perfectly baked and is being used However, I'm realizing that I can't…