I think where we get ourselves into trouble is by considering most decisions as dichotomous. You’re either for or against a thing, there’s no room for middle ground. So someone that moves from one side to the other can be labeled as unprincipled by some or as “finally seeing the light” by others.
The reality is that just about all of our positions exist on a continuum. If you feel like the other side is absolutely wrong in their convictions, you’re not trying hard enough to understand their reasons. You can say that they’re misinformed, misguided or prioritizing different values, but everyone believes things for a reason.
“Flip-flopping” in your case is simply moving farther down a continuum of beliefs, probably because either your access to information or perspective changed. In a healthy society, we all make those kinds of shifts constantly, and such movement should be celebrated. It shows that you’re examining your own beliefs and are empathetic to the positions of other people.
The less forgivable “flip-flopping” is understanding the attitudes and logic of people on multiple points on the continuum and simply parroting back the rhetoric to gain support. We make it necessary by demanding ideological purity from politicians and public figures, but it shows a single-mindedness that terrifies me. I should be able to disagree with you on an issue, have a healthy discussion, find a compromise and move forward. When we refuse to believe that someone we disagree with, even slightly, is at all capable of doing anything to help us, we’re begging to for them to lie to us.