I’d like to just save the image, burn it to a new SD and plug in it into the new PI (SD card form factors differ so I can’t just move it). What am I missing?
Assuming you are running the PI headless thus on the network, using the image method will result in a brief period (before cmdline.txt modification) where your PI B+ or 2 will have whatever MAC is in the .img’s cmdline.txt file if one exists otherwise it will naturally have the MAC native to the new unit.
If you have a USB SD card R/W module that accommodates the micro SD for your PI B+ or 2, you can clone the B’s SD card to the micro SD.
I have used this script with great success. https://github.com/billw2/rpi-clone.
(I came across this when trying to put an .img file on a SD card of the same size but it would not fit because the new card had slightly less capacity than the original one.)
With the MicroSD card still in the old B’s USB port, you can mount and edit the microSD’s boot/cmdline.txt file then move it to and boot your B+ or 2 with it.
The script will take care of mounting the MicroSD for you for the cloning process but you will have to mount it again to access it for editing. If you aren’t familiar with the procedures for mounting “disks”, you can easily become an expert with a few Google searches.