if you're someone like I was who was on 9.2 w/ game saves and eshop content you didnt want to lose, do this.
1. back up your SD card to your PC. afterwards put sd card back in the 3ds
2. boot gateway and Format Emunand. this will reformat the sd card
3. take out sd card. copy back up from PC back over to the sd card. overwrite any same-name files
4. with the sd card still out of your 3DS, reformat the 3DS and go through the initial start up steps till you're back at the dashboard. this will unlink your account.
5. turn off, put sd card back in.
6. turn on, note you have empty reformated dashboard (this is your sysnand)
7. boot into gateway mode, note you have your 'old' dashboard with all your old eshop games and stuff (this is the emunand)
you can then boot back to the reformated clean sysnand dashboard, perform the firmware downgrade from 9.2 to 4.2 and then when you boot back into Gateway mode either with the web browser exploit or the blue card, you'll be back to emunand with all your eshop content.
I'm no expert at 3DS, Gateway, or writing guides but these are the steps I took and I can still launch my eshop copy of Phoenix Wright and load my old 9.2 gamesave when I boot into Gateway mode. I recommend Googling and finding better steps then what I wrote, but just know that it is possible!
EDIT: i'm running into problems getting into the eshop with emunand! so might want to hold off... although my biggest concern was losing my saves...will update once I figure out what's going on.