Urban fantasy-ish YA series (found!)

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 This is a series of YA books (I think three books, but maybe more?) set in a more or less normal "present day" world, where a small minority of children are able to shapeshift into animals. At some point in their teens (on a birthday, I think, but I don't remember at what age) they lose the ability and get stuck in whatever form they're in at that moment. Some of them choose to become animals permanently. There's a lot of angst.

The plot of one of the later books revolves around some of the shapeshifters discovering that they can take the forms of supernatural creatures, too-- unicorns, phoenixes, vampires. But these creatures aren't really meant to exist and getting stuck permanently in a supernatural form messes with the balance of the world somehow. I think what happens is that someone takes an "evil" form permanently (a vampire, I think), and that forces someone else to stay in a "good" form permanently to balance it out? But I don't remember for sure which happened first.

I read these in the mid 2000s in the US.