If you install a new simulator, you need to re-run all our installers, you can't just copy files, because the installer also needs to create the proper add-on.xml for the new simulator.
Whether you need to download a new installer or not, depends how old your installer is. If you downloaded the installer before P3D V5 was supported, you need to download a new installer. Instead, if your installer already supports P3D V5, there's no need to download it again, because the FSDT Live Updater that runs at the end of every installation WILL update the scenery to its most current version anyway, but you still need to RUN the installer, to enable the scenery in the sim with a correct add-on.xml.