Yes you can update with a Poochy and Yoshi rom (hold Up on D-pad when booting into Gateway mode from the Gateway menu to turn off update blocking).
However you would be much better off doing these things in order:
1. Follow
https://gbatemp.net/threads/importing-gateway-saves-to-cia-games.425743/ except don't worry about importing the saves to cia games yet. Just backup the JKSM folder to your computer for safe keeping.
2. Follow
https://3ds.guide from the beginning and follow the optional "Move emuNAND" section when the option is presented to you.
3. Either convert your 3DS games to cia using GodMode9 (see
https://3ds.guide/godmode9-usage) or download using freeShop (I added a little tutorial to the OP of the Noob Paradise thread)
4. Finish importing your save games from step 1.
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The QR code option is inside the Network Install menu item.