It's way easier that it seems:
just install the FSDT Universal Installer and it will update into your existing "Addon Manager" folder, which was already setup to NOT be placed "inside" P3D ( unless you manually directed to it, something we always suggested against ).
From the Addon Manager folder, the Updater will be able to handle all the FSDT products that are already there and, in fact, the FSDT Live Update is still there, just with a different look. All products for MSFS will stay in the "Addon Manager\MSFS" folder, and whatever code or files they share with P3D ( GSX for MSFS shares lots of files with GSX for P3D ), it's not something you need to be concerned with, the installer and the updater knows what to do.