I think the answer to the question "why the change in opinion" has to do with the polling mechanism: Phone calls. Please remember that by 2009, the date of the last poll, that it was AFTER the housing crash that continued to put people out of their homes even after the election in '08. I wouldn't be surprised if the polling was a bit skewed by who was more affected in terms of home displacement, specifically that those in a more transient housing market would be LESS likely to have a permanent land line. I think that is significant not because financially strapped people value life less, or anything like that, but in the case of an unwanted pregnancy, they might be more likely to choose abortion than richer people who may be able to absorb the cost of another child with a little juggling. That is, to some extent, I do think our choices are influenced by the resources available to us.
Additionally, we Americans have a very short attention span. Now that it is 30 years since Roe v. Wade, who remembers their peers having been forced into involuntary pregnancy and labor? I mean, I know my mom was terrified because she was brought to a Catholic hospital in '72 (four years before me) where the staff was going to force her to have a baby whose limbs had been partially expelled at five months because they would rather my mom have died of sepsis than the severely injured baby perish, but who of those polled have things like that in mind? (Re: my mom--the staff insisted that she keep very still; their plan was that she remain in bed rest to save the baby in spite of the fact that there was a 90%+ chance that she would suffer from infection and early labor would have to be induced. Knowing this, my mom did every calisthenic she could think of until the baby fully aborted. You might think it's terrible, but my mom lived and produced two fully healthy children afterward and I am very glad of her choice.)
As to the issue itself, I don't think many women take the question lightly, no matter her circumstance. But let's just say for the sake of argument that one woman did--that she cared about as much for her fetus as she does for her feces: isn't that precisely the kind of woman one would hope would never become pregnant in the first place? And say we forced that woman to go through with the pregnancy. How prepared are we to ensure that she takes good care of the fetus? What if she doesn't want to surrender it when it's born? What if she does give it up for adoption but it's damaged because she used drugs? Are you going to adopt it? Be real.
I am pro-abortion, to strip it of "nice labels." Would I have one myself? Really, it depends on the circumstance. Although, I was pregnant by rape in a strange city living with my rapist and decided to have the baby anyway knowing that there were good people in my acquaintance willing to raise that baby. But that was largely because my parents who were ready to disown me right up until they found out I was pregnant relented and continued their COBRA coverage of me. If I had no health insurance, would I have gone through with it? Probably not.
Oh, and I don't think that the opinion that women shouldn't be stripped of their right to choose abortion is Libertarian. The Libertarian goal is devolution of power to the State and even municipal level. That's not the same thing as protecting a woman's right to health care and abortion if she elects it given her health care options. I think that a woman has a right to determine her own choices regarding her fecundity and no one should bar her from making that choice. Only Federal protections can guarantee her that.