You won’t lose the exalted status of Honorbound when you faction change. But you also won’t automatically have exalted status for 7th Legion. You’ll need to build that rep up after you faction change. And as long as you have a Horde level 120 and an Alliance level 120 with all of the requirements for each faction, you’ll be able to play ZT and KH when they become available. Once you earn an achievement, you don’t lose the achievement. Once you complete a quest, you don’t lose the completion of that quest, no matter how many times you faction change. They’re just not transferred to the counterpart achievement / quest on the opposite faction.
Edit: I just read a blue post that says you will automatically have exalted status for the opposite faction (i.e. 7th Legion) rep.