Ok to do this, you need to be Exalted in AV BEFORE you faction change. When you faction change, your Stormpike insignia becomes a Frostwolf insignia, but you get to do all the AV quests again as a Horde member (this probably also works in reverse). Do the Frostwolf Banner quest and you will get rank 1 Insignia. You should then be able to immediately rank it all the way up to rank 5. Then you will be able to equip and use both the rank 6 and rank 5 insignia, giving you two seperate hearths to the Frostwolf Keep while in AV.