I had to do that with several airports after I installed ORBX LC but found that also VECTOR changed the elevations that several airport AFCAD's rested upon (that is why that file is below the AFCAD). I had two choices: first was to find out if ORBX had its own designed airport AFCAD. If so, I normally found it easier to disable it (change BGL to OFF) in the ORBX directory. I found, however that disabling any of the elevation BGL's for an airport caused major distortions in the airport and the scenery. So the only solution was to load the default airport into ADE (scruffyduck). I then loaded a default 737 and set it on the runway of that airport. I know that the elevation top menu line reads 10 ft above the airport elevation. So I loaded the default airport into ADE and in ADE/tools menu I selected "change airport reference data" and set the new corrected airport elevation. I then saved and compiled my corrected airport. Compiling it placed it in "addon" scenery file so it was now above the original default. After restarting FSX all my scenery elements at that airport appeared to be in sync since GSX2 reads that top most AFCAD.
The key is that you don't have to "create" a new airport, just load the default, change the field elevation in the AFCAD, and then recompile it. Now, I have FSX, but the same technique hopefully will work equally well in P3D.
Craig