Changing a 3D file's extension does not convert it. Each format carries different data:

• STL stores triangles only: no materials, textures or units.
• OBJ keeps materials in a separate MTL file and textures as image files.
• GLB / glTF can hold a whole scene with PBR materials and animation.

Before sending a model to a printer or a web viewer, check what the target format can keep, then re-open the exported file to confirm it still has the triangles, dimensions and materials you expect.

I use 3D Converter for this. STL, OBJ, PLY, 3MF, FBX, GLB and glTF convert locally in the browser with no sign-up, and the tool re-opens every export and shows a before-and-after report (dimensions, triangles, objects, materials, textures, units and axes). It does not repair meshes.