You're also the 10,000th person in the past month who updated without a backup. don't feel too bad.
Ok, so when you are in system nand and launch the ds profile hold down L and the gateway menu will open. In there select "backup nand" after its done running, take your big SD out and stick it in your computer. Copy EVERYTHING off of it, paying attention to the 1gb or so .bin file (maybe nand.bin). That's your system main backup, it requires a hardware mod to restore but people here do the mod. If you experience catastrophic failure it can save you, keep it somewhere safe. Put the sd back in the ds, hold L and launch the ds profile. Select format emunand, it will delete everything off the sdcard and start you with a fresh 4.x emunand. A copy of smash bros will have the 8.1 update, so obtain one of those and put it on your gateway. Launch back in to the DS profile holding only UP on the d-pad. Make sure wifi is turned off. Now when you try to launch smash, it will update to 8.1. Remove your big SD, search this forum for emunand tool. Delete everything on the bigSD, and copy everything from before back on it. In emunand tool click extract emunand. Now you should have your nand backup, emunand backup, and any eshop stuff should show back up (no promises on that one)