the complete record for the airport is needed with your changes... the reason is because FG needs to load the definition of an airport all in one go... so when FG finds your complete definition of VHXX, it'll load that first and ignore any others...
you can put the complete definition with your changes in a file (eg: VHXX.dat) which you then place in a custom scenery directory named NavData... NavData contains three directories... they are named "apt", "fix", and "nav"... your data file would go in the "nav" directory... so in the end you might have the file in "/home/your_user/MyFlightGear/MySceneryOverrides/NavData/nav/VHXX.dat"...
then you add "MySceneryOverrides" to FG as an additional scenery directory... this is easily done in the launcher's Addons tab... if everything is right, the next time you start FG, it should rebuild the databases... if that completes with no errors, then you should be able to fly and check out your changes in-sim... any changes you make to your VHXX.dat file should be seen the first time you start FG after making them... any new files for other airports should also trigger FG to rebuild its airport database... some custom scenery comes with a NavData directory these days
"You get more air close to the ground," said Angalo. "I read that in a book. You get lots of air low down, and not much when you go up."
"Why not?" said Gurder.
"Dunno. It's frightened of heights, I guess."