mdd,
I cannot find the sources for what we typically think of as tznius standards (I can’t find my notes), but my recollection is that some of our tznius standards are regular takanos d’rabbanan (I think the body falls into that category) while others are “only” daas yehudis. The latter category are the customs of frum women and I don’t see why they can’t change both l’chumra and l’kula.
A minimal hair covering according to almost all poskim is not daas yehudis but d’oraisa (or at the very least d’rabbanan).