Create a batch file in notepad with the following command.
imagetool -nobeep -dxt5 -dds -mip -nodither *.dds
Save that in the texture folder containing your dds files and make sure you have a copy of imagetool.exe in the folder too. Then run the batch file and watch it do its magic.
You can change -dxt5 to -dxt1 or -32 (32 bit) for differing levels of quality, though if you're converting .dds dxt5's to 32, you won't see much difference due to the compression of the source material.
You can use -mip for mipmaps, but for aircraft I recommend -nomip to ensure you don't get the dreaded blurries!
-dds basically means you're saving it as a .dds extension
As a further note, Imagetool batching will also handle PSD files and BMP files too, just alter the last element from *.dds to *.psd or *.bmp
Whilst this isn't the only way to do batch conversions, I use this format for converting my aircraft PSD masters to dds for flightsim.
Hope that helps!
David.