a factory reset removes the black/amber nokia updates, your only choice is to downgrade to wp8, reinstall the amber&black updates and upgrade to wp8.1
it takes about an evening but it is worth the effort
search the wp8.0 forums on here for an easy guide written at the time of gdr3 for people to go back to gdr2 using that nokia software for pc (i think it was called something like caresuite)
keep us updated if you run into trouble