PKHeX/PKHeX.Core/Resources/text/script/gen3/flags_frlg_en.txt at master · kwsch/PKHeX
pokefirered/include/constants/opponents.h at 70b76a15df8c6a6dd03d7a09e8f9eddd2e4dc29d · pret/pokefirered
(opponent flags start at 0x500, so 1718 - 0x500 = 438)
pokefirered/data/scripts/hall_of_fame.inc at 70b76a15df8c6a6dd03d7a09e8f9eddd2e4dc29d · pret/pokefirered
It's automatically cleared when you enter the hall of fame, so you can consider it a "temporary" flag.
replace the old patched rom with the new one. as long as it's in the same place with the same name, the save will be usable.
alternatively, some emulators let you directly import a save file from an older version
also, latest bugfixes are live!