As has been said, GW is working with BootCTR9 (seems fitting, being that's what it worked the best with when A9LH v2 came out as well). I did have to convert the GW payload with firmtool, but, it works great. So, my biggest concern is already addressed. Granted, I have a Sky+, so I was never in a real bind anyway. But I'd miss the menu. And the cheat engine (though it doesn't look like any Binding of Isaac expansions are coming to the 3DS, so I probably won't be needing it in the future).
There's still one thing I would like to see for B9S that I can't seem to find anywhere. Namely, a Safe A9LH Installer. I know, that seems like going backwards, but there are several reasons some people might want to go to A9LH from a fresh B9S install. Key among those, atm, being that it's a more mature exploit that's not going to be receiving a lot of updates, thus not breaking things with new releases, not needing a re-flash every couple of weeks, etc. There's also payload compatibility considerations. Salt FW just completely doesn't work with B9S, for example (though pretty much everything else does). Old .3dsx homebrews tend to be incompatible with the new launching method as well. Not all of these are still in development, so, it's impractical to assume they will all get rewrites. I'm sure that the Luma team will improve .3dsx compatibility in future releases, but for the time being it has a way to go. Never mind the minimalists that will just never go for an integrated ARM11 core. Besides, it would probably just ease people's minds in general knowing that the process can easily be reversed without performing a NAND restore.
I can't imagine it being much more complicated going from B9S to A9LH than it is going from A9LH to B9S (especially with B9S having OTP access). I get that it's not something that's in heavy demand with everything going to B9S now (and rightfully so with it being the superior exploit, being much easier to install initially, having ARM11 at boot, etc.) Still, it shouldn't take a NAND restore to reverse the process should someone want to. Besides, the developers that are still supporting (or trying to) both exploits clearly NEED this, so they can check their .bin builds. The latest Skeith freezes before loading, and the latest CBM9 has a regression that can cause it to not display it's menu in no-init versions of A9LH.
Also, my apologies if this is considered necro-bumping, but the thread's not quite a month old yet, and it's one of the first things that pops up if you Google "B9S to A9LH" so, it only made sense to request a link where people are going to be looking for it. And thanks in advance for all helpful replies.
