Welcome to Simviation..

Each ai ac is defined in the ai traffic configuration file (traffic030528.bgl in FS9) by its name and texture name if more than one.

When you replace the textures of an existing ai ac it uses the already configured AC# in the ai traffic configuration file.

When you add a new texture you have to add the 'new' ac to  the  file using Traffic Tools V 2.02, here http://www.simviation.com/fs2004utilities1.htm Instructions are included.

If you add it by replacing one of the existing AC#'s your new ac will appear in its place. If you create a new AC# for your ac you will have to create new flight plans for that ac. (You'll see what I mean)

Cheers,
Dave