If you set up menuhax, and then change firmware (downgrading or upgrading) you'll have the same problem since the payload won't match the actual firmware you're on. Once you have reinstalled menuhax for the new firmware you won't have to install it again (unless you upgrade/downgrade again).
--------------------- MERGED ---------------------------
The guide will still be at Plailect's GitHub, the same guide you used for parts 1 and 2, but the rest of the parts will change. If you want a9lh within a week you should just do it now (copy and paste the text of the guide, and download all the files linked and put them in folders on your computer called Part 3, Part 4 and Part 5, in case anything changes while you're doing it). The new guide was announced as coming "in a few days" more than a month ago so I wouldn't be holding your breath waiting for it.