Before importing your files, please pay attention to whether your files are already linked to your Lightroom catalog or not, e.g. they come straight off your camera.
This is very important, as Lightroom doesn't allow substituting old files with newly compressed files automatically and will force you to relink just 1 file at a time manually (apart from images from the same folder that were already in dng format,
see more), which makes it impossible if we are talking dozens of images.
To avoid this, please use "Optimize Lightroom catalog folders" feature.