Hi James,
I'm not aware of a specific tool in Terragen to search and replace/update file paths for image maps loaded into a project.
I suspect that if the image maps existed in the Terragen Library, the library would also need to be refreshed, otherwise they would point to the older location.
One approach might be to write a Python script that would use the Terragen RPC feature to read through the image maps in the project, and replace existing paths with another path that you would specify. Another alternative would be to text edit the project file, replacing the old path with the new path via a copy/paste operation.