I was also wandering.I have some private server client version 6.0.3. Is there a way just to use this launcher from official site and update game client to 8.0? I remind i have 10gb limmit and i dont know if its enough to start fresh download.
After you've downloaded and opened that launcher long enough for the folder structure to be generated, cancel the download and copy files from "data/Packs" inside your 6.0.3 version into "data/Packs" inside the folder generated by this launcher. Then re-open the launcher and wait for the download. Should save significant amount of traffic.