From everything that I have read, and trust me, I have read an awful lot, what you have prior too hormone therapy, will stay where it is at. Any additional fat that you recieve AFTER hormones are introduced to the body, will get distributed toward the female areas. Now if you are trying to lose weight, again, from what I have read, is that it will take from where the male fat was, to ensure that the hips, buttocks and thighs maintain the weight due to the biological female trying to add cushion to that area for pregnancy. Like if they fall, or what not.
As for buying new pants, I wouldn't sweat at this point in time. Wait and see what happens first. Since you just started, I would give it some time, and see how your body responds. Just eat healthy, don't gorge your self, and you should be fine....add spicey foods to your diet, to help boost your metabolizism, as estrogen has a tendency to slow down metabolism in your body....