I don't know about your health insurance, but on mine (Kaiser Permanente, in case it's locally relevant to you) all they need is your driver's license to declare you male or female. Also, it's not like your medical record just disappears when you change legal genders; if you're on T and have been diagnosed with gender dysphoria, they're not going to just go "Well guess you don't need those anymore, you grew a prostate/penis/male reproductive system, right?" I'd say go for it, it'll make things less confusing down the road.