So, found this hint after a quick search, http://serverfault.com/questions/488092/binary-zone-file-on-bind9 the binary zonefiles can for instance appear when doing slave transfer between master and a slave node.
If you then need to upgrade the slave node to master, converting the zonefile from raw to text is what you'll want to do first.
# convert raw zone file "example.net.raw", containing data for zone example.net, # to text-format zone file "example.net.text" # # (command) (format options) (output file) (zone origin) (input file) named-compilezone -f raw -F text -o example.net.text example.net example.net.raw
Not sure when you'll want to go the other way, but it's very similar
# convert text format zone file "example.net.text", containing data for zone # example.net, to raw zone file "example.net.raw" # # (command) (format options) (output file) (zone origin) (input file) named-compilezone -f text -F raw -o example.net.raw example.net example.net.text
Then of course, you have to edit the relevant zones in named.conf telling it that it's no longer a slave server but a master, but that should be fairly trivial if you're running a master/slave setup to begin with :-)