I've tried several methods and am out of ideas now. I've tried changing the spell effects of Lady's Favor and Lady's Grace in the CS, I've tried player.removespell, and showbirthmenu. They either have no effect or, in the case of showbirthmenu, reset my spells (which removes all my buffs).

Any methods I haven't tried yet??

EDIT: fyi, the reason I want to switch birthsigns now is because Morrowind calculates the +25 endurance from The Lady as a part of my natural Endurance, rather than as a buff, so I'm capped at 100 instead of 125. I switched Medium Armor for Heavy Armor in my Major Skills, so as I'm training it I'm leveling purely off an Endurance skill, but I have no room to grow my Endurance so I only get three +1s! /wrists

Basically, all I'm trying to do is switch The Lady birthsign for The Warrior.