-a option works like -A option

[C0rn3j]

Log: http://i.imgur.com/Lvcey8I.png

I'd imagine the -a option would keep the last 3 HEX blocks, instead of changing all 6.

[thoger]

Neither -a nor -A are supposed to preserve vendor part of the MAC and only change the last 3 bytes. That's achieved with -e.

macchanger comes with 2 OUI lists - OUI.list which includes all reserved vendor prefixes, and wireless.list which includes few prefixes that were used for wireless network cards. If you use -a and your current MAC is on the wireless list, macchanger will pick one of the vendor prefixes form the wireless list, which may be different form your current one. -A allows it to pick prefix from either list.

FWIW, I believe wireless.list has not been updated for quite a while and hence seems rather useless these days when devices are more likely to have wireless than wired network connection.