edited default.prop (ro-secure=0...) and packed it again.
Everything was fine, no error or so.
But the sizes of the original and the new image have tremendous different sizes!?!
I don't dare to flash them. The boot.image of the attached screenshot was taken from a backup.
I tried the same with a copy of the kernel made via flashyfy.
The size changed from 8.305kb to exact the same size as you can see in the screenshot.