Lets get real - changing your sex - the operation, the years before the operation taking hormones, and all the heartache, is not something someone is going to do simply to improve their chances of playing college or pro ball. There are also years of counciling and pulling off a fake desire is not very likely.
And while height is certainly an atribute, the loss of muscle mass due to the hormones probably makes controling that height a lot harder.
This is never going to become a trend - it is too rare to combine the athletic competitiveness with a desire to change sexual identity and the transition time is so long that most of a career would have been forfitted before it was completed.
There has already been a pro tennis transexual playing professionally without much success by the way.