follow this guide https://gbatemp.net/threads/guide-change-emunand-region.392099/page-10
It worked for my new3dsLL 9.1.0-20J to 9.2.0-20U (EmuNand only works in CFW)
(Download updates from 4.2>8.1.0-19 old3ds +new3ds 9.0>yourfirmware)
1.Format Emunand
2.Inject FBI H&S
3.Install sysupdater in...