Most effects are reversible, doubt they all are, though. Breast growth, for one, isn't. I've started budding in about the third, fourth week on HRT. It's somewhat dependant on how receptive your body is towards estrogen, and how old you are (I'm 22). If you're going to be on DHT blockers and anti-androgens, that might do damage as well (they can have some strong side effects).
HRT isn't something to be toyed with. Even if you don't feel any different, there are major changes going through your body as it starts to process the drugs.
I would suggest reading about anti androgens, DHT blockers, and estrogen on wikipedia and other trans websites concerning health.