Default [Q] Upgrade from MI1 RDLV - Howto?

Hi, I currently run ATT MI1 firmware rooted with de la Vega. I would like to upgrade to MI9 or even better to MJ5. I thought of unrooting and then to try to flash back to ATT stock MI1 firmware, but I cannot find this old firmware....
Maybe U can help me find a better solution without risking to brick my GN3....
Many Thanks.