Problem solved. I bit the bullet & installed Adobe AIR. As I suspected this associates the .air file extension with the new software by default.
I took this shot of the FS9 default C172 folder before installing Adobe AIR.
Note the icon showing that AIR files are associated on my system with AirEd which I use for editing these files.

This was taken after installing Adobe AIR.
Note the new icon which shows that all AIR files on my system are now associated with the new software.
This is the same with every single FS/CFS aircraft on my HDD.

I fired up FS9 & selected the C172 to check it out. Note that it works perfectly. It would not load if the file was corrupt.

The only problem would be if you tried to open MSFS AIR files by double-clicking on them. They are obviously not compatible with the Adobe software so it will think the files are corrupt.
This is the warning message which I suspect caused this rumour to circulate.

This confirms my suspicions that there is nothing at all to worry about. Leaving it as it is will do no harm whatsoever but I find it more convenient to re-associate the AIR file format on my system with AirEd. This is a straightforward procedure taking only a few seconds.