I installed P3D v5.3 tonight and had quite a few test flights:
as far as I can tell, all my Orbx sceneries work properly (openLC, airports, etc).
As advized, I completely removed P3D v5.2 (see post of Rob Ainscough hereabove).
Since my Orbx sceneries are installed in a library, in theory all I needed to do to get Orbx working in the new version was : open the settings in Orbx Central, then go to the Help section, and click Sync Simulator. I did that, and just to be sure I also uninstalled and re-installed Global Base Pack (I think it was needed in the past).
That's it.