It's a bit late for that, I'm afraid. To get that arrangement, you'll have to forgo BootCamp Assistant (you don't actually need it, you know, it's just a convenience) and reinstall Windows, because your NTFS partition can't be (easily) resized. If you don't want to touch Windows now it's installed, you could also sacrifice space on your OS X slice to install Yosemite on it, and this should be possible, again being a bit complicated and again losing BootCamp Assistant to revert.
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