I don't have a hard mod. Just taking educated risks. I used the known safe methods of getting to 2.1 (I remember the SysUpdater horror stories, and wasn't about to try the beta version of Time Machine). So the only risk I took was that the A9LH installer and NAND restorer would mess things up. GW's NAND restoration code has been solid for a long time, so this seemed unlikely. I also have Sky Blue and Sky+ laying around, so I'm maybe a bit overconfident, having every trick in the book at my disposal. I took another educated risk again, updating to 9.9 with PSMD, when none of my NAND.bin files worked, after reading that firm protection was not enabled, and that people were getting factory 11.0 (and not getting a brick) when updating online. It worked, and I was then able to downgrade, and restore my original A9LH SysNAND.
Don't mess with it. There are enough people who have FIRM and NAND dumps to provide them to those that can make good use of them to provide us with a working alternative should GW fail to provide a .bin file. It's not worth risking getting stuck. Especially if you don't have a Sky+ loaded up with PSMD and CN ROMs to bail yourself out.
In my case I just installed NATIVE_FIRM from 9.3 via FBI and then downgraded with browserhax, but our methods were basically the same in the end.