To be a hypocrite you have to DO something. You can change your mind on lots of things and not be a hypocrite, it's called changing opinions based on new and convincing evidence.
But;
Specifically preaching against doing an action, and then performing that action yourself, is hypocritical. Preaching against gay rights and then having gay sex is hypocritical. Protesting a seal hunt, and then participating in the seal hunt is hypocritical. Protesting abortion, and then having an abortion is hypocritical. It stops being hypocritical when you stop holding a view against "something" while performing that "something".
Just "changing a belief" is the tip of the iceberg. You're absolutely right, views change as we learn and get older; but specifically denouncing act xyz, and then performing xyz is the basic definition of hypocrisy. So, if after the abortion you became pro-choice you would STOP being hypocritical, but until that point where you drop the dissonant view, you'd still be a hypocrite.
See what you say is all well and good until you actually find yourself in the situation of your missus being pregnant. As far as Im concerned you've no right to judge a person for changing their views when presented with the aforementioned scenario, as it is pretty lifechanging. If they have an abortion and continue to be pro-life then they are a total hypocrite, but if they suddenly become pro-choice then they're just "enlightened because of life experience".
Your example about the gay sex thing, a person can think they're straight, preach against gay rights, try it out for themselves and decide its what they actually like...are they a hypocrite? No they're not.
But...
If they tried it, liked it, decided they were gay but preached against gay rights then they're a hypocrite.
So as such, you really cant decide on abortion until your forced to make the choice for yourself. Even if you said it was wrong beforehand, its such a huge life event that if you changed you're mind then thats your choice, just as long as you dont continue to preach against it.
Realistically EVERYONE should be pro-choice, its noones right to tell someone else what to do. If you dont like it then dont do it, but dont expect other people to conform to your world view.
And without meaning to bring religion in, most of the pro-lifers are Christian or at least were raised in a demographic that is heavily influenced by Christianity...which I suppose is typical of such a religion, because most try force their shit down your throat daily.
Anyways, point is
noone has the right to judge another person, especially not if they havnt been presented with that choice to make themselves.
Example in point, I could say I dont like rollercoasters, they're dangerous and scary and I dont agree with them because you could kill yourself on one...but if Iv never tried it, and try it and decide I was wrong and that I like it, am I a hypocrite? No Im sure as fuck not.