Just be sure you download their latest installers, which all support P3D up to the current V3 version.
If you have them already activated on your current Windows installation (because you activated them from FSX), there's nothing you should do: they will just work as full versions in P3D too.
If, instead, you are using P3D on a new system or after having reinstalled Windows, so you never activate the sceneries on this new configuration yet, you will have to use the "Register Serial" option of the Addon Manager menu inside the sim, and reactivate your Serial Number.
