Ok first of all you need to have been previously permanent ghoul spec. also im unsure if this works in the alliance equal town.
If your Ghoul dies you need to respec ghoul perm and then back to blood or frost and repeat the exploit.
Now you need to go the Agmar's hammer in Dragonblight and find the stable master just outside :
talk to him and tell him you want to make use of the stables, you should get an error looking like this (maye not the first time, cant remember):
next you need to drag your current pet (looks invisible at the moment) into a stabled slot (will take shape of an icon once you start dragging):
Now simply drag the stabled pet back to your current pet:
Press okay on the error whenever it comes up, close the stable window and dismount, their you have yourself a permanent ghoul in a different spec:
This last screen you can see it it real because the ghoul is an actual pet not a summoned thing with a timer as it appears next to portrait.
Tried to make it as idiot proof as possible hence so many SS's sry if it makes the page huge.