Verified answer

Yes and No.

Some people have had 'sex change' treatment, and then realised that they were not, in fact, transsexual, and attempted to reverse the process, but it's rarely very successful; which is why there are so many checks and safeguards built into the process to try to prevent non-transsexual people going through it in the first place.

It's possible to partially reverse the effects of the hormones therapy;the effects of oestrogen will reduce if you stop taking it (not by much, though, if you've had your testicles removed), but some of the effects of testosterone (facial and body hair, deepening voice, et.c) are permanent.

It's possible to partially reverse the surgery, and to refashion the genitalia to a degree, but it's very unlikely that you will ever achieve a functionally normal penis again. and of course both men and women, post-surgery, are sterile.

I'd add one thing to Diane's answer.......those who decided they made a mistake weren't transsexual to begin with but became one through surgery