The short answer is that we did it because it was the best decision for our family. That's really the only answer that matters. School shopping is not a one-size-fits-all deal - everyone has different needs, and those needs will be met by different schools. And just because a school was the right place one year, doesn't mean it's still the right place three years later.
But the long answer is that their old school in June 2014 was not the same place I enrolled them in in September 2011. You know that story about how if you put a frog in cold water and then turn it up to boiling, it won't notice and will get cooked to death? Well, that's a myth. But this experience with school was like that - change happened so gradually (and I was more out of it than usual because I'd just had a baby) that I didn't notice until the school year was almost over. And I all of a sudden realized I wasn't happy with the school I once loved. It was mostly to do with two factors: a change in leadership, and a rapid increase in enrollment, all at the same time.
My worries about the school going through a rocky transitional period were confirmed when I looked through some old papers and found school circulars from previous years. What a lovely place they told of! What a sense of family they fostered! What evidence of attention to the children's education they were! And they told a story of a completely different school than the one my kids were currently going to.
However, enrollment periods for private schools here had long passed - by the time I was serious about switching them, it was June, and the assessments are done in January/February. So it was a small miracle (actually kind of a big one) that when I called the new school, they had spots (!) in both grades (!) and time to give the assessments (!), which are quite difficult, and both the girls passed (!).
I am still sad almost every time I think of their old school and the teachers, friends, and great experiences we left behind there. But the girls are doing well and so far, we are really happy with the decision to move them.
I have a feeling that their old school will be back up to snuff in a few years. And for a while, I almost wanted to stay and work through that transition with the school, to be a part of positive change and have an influence on the direction the school was going. And as hard as it was to bail out on that opportunity and change the girls to a different school, I think we made the easier - and for us, the right - choice.