Nothing has been announced yet, but unlikely to be a copy/paste due to the current version requires battlenet to launch. Even some of the consoles are requiring reinstalls for shadowkeep.
Regardless of the launcher most of the files will remain the same. If we can copy and paste the "packages" folder into Steam that will save us a 87 gig download.
They would need to release an update through Battlenet to be compatible with Steam. Destiny 2 would need to pull the Roster from Steam now and not Battlenet.
The launcher has nothing to do with the package files... and the package files contain the game you see on your screen. I decided what I am going to do once we are given the go ahead for steam, is that I'm going to start the steam download and let it generate the folder structure. If it generates a package folder, I'm going to stop the download and copy all the *.pkg files over and then restart the steam download. If it works, I'll have saved myself hours of download time.
After doing that verify the cache and it should download it change any missing or incomplete files