Don't mix up .sh3f and .sh3t files. .sh3f is a furniture library and .sh3t is a texture library, two completely different types of libraries.
You import .sh3f files with "Import furniture library" and you import .sh3t files with "Import texture library".
When asked to Replace a library it basically means that the old one is removed and the new one added. No other models or textures are touched.