I am struggling to see why you can't understand this?
It's not me that can't "understand" you. It's your own description of the problem which was entirely inaccurate. First, you said:
redownloading the installers only allows me to install into p3d v4.
What other means can this sentence have, other than the installer not detecting P3D ? So, is this true or not ?
After the automatic migration that your sceneries have done when installing aproduct into V4. I haven't got anymore FSDT in V3.. How do I fix this?
This description is very vague, because it's related whether your first description of the problem was accurate or not. If it was true that "the installers only allows me to install into p3d v4", it's entirely NORMAL and to be expected you wouldn't see the products in P3D V3, since the installer didn't presented you the option to install there.
So, again, it all depends by the installer being able to find P3D V3,
even during the migration. If it didn't see P3D V3 when you migrated the scenery, and this was only related to the registry wrong, the sceneries were moved, but not enabled in P3D because, obviously, the migration will enable the sceneries only the simulators
that were found to be installed at that time.