My headcanon for Percy is that the Hat gave him a choice, and he chose Gryffindor because it's what was expected of him. He's so rule-bound and intense about doing what's proper, I can't imagine he'd even consider going off the beaten path by picking Slytherin.

I think you're right about Ginny, it wasn't so much that her personality drastically changed in OotP, more that she didn't have much of an established personality before OotP because she was such a minor character. Of course as fans we extrapolated as much as we could, but there wasn't that much to go on.