I think it's as easy as taking blank SD card, and copying all the files from the original iFly SD card to the new one.
I don't know if the dedicated iFly units still support the "Streets" navigation app. (I have never used that feature), but if yours does, then that feature will not work with the second SD card using this method (it is somehow "locked" to the original SD card...I dunno how they do that). Sounds like that may not be important to you, though, since the device always stays with the plane.