David Paul wrote on Dec 13^th, 2011 at 7:10pm:

If you are just talking about changing between the clamshell reverser engines and the ones with the internal reversers, you can do that by changing the "model=" value in the aircraft file for the livery you want to change.  In some cases, though, you will also end up with winglets.  But the /F models you change "model=f" to "model=f_re" and you will have the clamshell engines.

Awesome! Thanks David.