Well, like I have written above, I have sound reasons to think that nothing much would happen at all, as you are suggesting

But you are right, there is a motive: what if I would be the 1 in 1,000,000 for whom it could actually do something? Even though I know it's silly, it's mostly harmless as well, so why not explore this, so that I can say to myself "I have tried everything".
Now that I have stressed that I know it's highly improbable, I'd add some technical comments. In general, having the right hormonal environment within the genital region is all one needs for it to develop in male or female direction (in the fetus). Unlike the development of the gonads, it is not dependent on your genes (XY/XX). Quite significant changes can happen in the genitals even after birth, during puberty: in guevodoces (5-alpha-reductase defficiency), the genitalia appear female until puberty, then the surge of teststerone transforms them into male (or more precisely, finishes the transformation that should have happened before birth). So mainly, I think the reason why nothing will happen when applying estrogen cream to pre-op genitalia is because they are adult. Adult tissue just wants to stay the same. Also, the body only seems to know how to grow new tissue and not how to "ungrow" any cells.