Here's the situation:
When you bring an image into SketchUp for use as a texture, a Material object is created. That object stores the original path to that texture file, so a location already exists. The free version will look in that stored location for reloading materials, and will reload it if it exists.
The pro version will let the user specify a folder to search for matching file names. I had not thought about this too much before, but I suppose each texture could be given a specific replacement. That would not be as fast for the user if they chose to do that, but it would be more flexible. The fastest/easiest way for the user would be to specify a folder for replacements.
Any need for exporting the images in a model to a folder? I know something exists that does that, but it could be added in...