Rather than try to hack knote which will inevitably break when it gets updated to resize it to A5 why not instead just resize the old notebooks to the Boox size? You'd only have to do it once.
You can do it pretty easily with the "mogrify" command with the free program Imagemagick. It's run from the command line so you have to run it in command prompt. The single line below will convert every single PDF in the current folder to be the same size as what the Note Air exports. Obviously make a backup before trying it as mogrify OVERWRITES the PDFs.
Code:
mogrify -density 72 -define pdf:fit-page=1404x1872 *.pdf