Is it possible to replace files?

In Lightroom, you want to work with the raw images because the raw images have the most accessible data and will provide you with the greatest ability to manipulate that data and obtain the best result. All of the changes are simply stored in the catalog, not applied to the image itself. Then you export copies that contain all the changes that you make. The original raw files are always left untouched, in their original pristine condition as they were when they were  downloaded from the camera.

On the other hand, there is an option where you can import your images to an external hard drive for instance, and then choose to generate smart previews which are smaller. When the external hard drive is disconnected then Lightroom will use the smart previews for editing purposes. When the external hard drive is again connected the adjustments will apply to the master images. The only disadvantage is that smart previews cannot be used to edit in Photoshop. The master images are needed for that option could be available.

In Lightroom you are never ever editing the RAW files. Not directly. You are manipulating a database that contains, amongst other things, your edits. The image you see on the screen is a small preview file, different preview files in Library vs Develop, but still preview files. The preview in Develop is larger and temporary (think purge manually or via closing lightroom)

Smart Previews  can speed some   things up, as the are smaller, and do not need access to the RAW files, except at creation of the preview, and exporting.

Can you provide what you are trying to do?