I think you could also rename knoppix-data.img to (for instance) old-knoppix-data.img and the contents would still be available for loop-mounting, if needed. The script that looks for knoppix-data.img won't find it, and thus will offer to create the persistent store when you boot the USB again, along with the encrypt option. Once the knoppix-data.aes file is created you can grab whatever you want off the old .img file.

Cheers!
Krishna