Something you may want to know. I'm trying to do the exact thing that you are, except I'm updating from 6.6. (Mainly so I can get mGBA to work properly.)
The directions will tell you to restart the chainloader menu after installing the new version of BootStrap. If you follow the directions and the chainloader menu stops working entirely, don't worry!
Skip that direction and go straight to swapping out the boot.firm file. Apparently, older versions of Luma don't recognize BootStrap as a valid entry point after it's been updated. Swap out the boot.firm file so you have the new version of Luma instead, and it'll recognize the entry point just fine.
Now, my problem is that GodMode9 doesn't seem to recognize B9S after the update, so the bit about running the cleanup script is absolutely useless. I think you should be able to update GM9, but honestly, I just went ahead and fired up Luma as-is at this point, and it seems to be working fine now. Everything seems to work, although I don't fiddle with the Rosalina stuff at all since I'm not entirely clear what kind of extra features are in there, and I remember that it caused crashes. To be fair, those crash reports are from way-back-when and B9S was just recently a thing. They may have, and probably have, gotten fixed by now.
