see https://forum.inaturalist.org/t/is-it-possible-to-merge-two-accounts/2234/4
i would assume that, as an alternative, you could also delete or rename your old unused iNat acount, freeing up that login, and then just rename your current iNat account to that old name. but i’ve never tried that before. so i would still reach out to iNat staff first to see what they think is the best approach.