If you're rooted you DO NOT NEED TO be on MA6 to go to MB1. Just follow the steps in post 2 of the MB1 thread linked above. That loads a complete, stock but rooted, ROM. It will upgrade you to MB1, which is the latest, regardless of what version you're on now.
If you're already on Jelly Bean (4.1) you can skip the Factory reset step, doing what's called a "dirty flash." Worst case, if you run into problems after the upgrade, you can do the factory reset then. You can see what version you're on now by going to Settings, scroll all the way down, and tap on About phone.