In Xcode, how can I utilize new UI APIs for Yosemite, but then revert to older ones for older OS'?
You'll need to figure out at runtime if method/class is available. Here's Apple guide about "SDK Based Development".

In Your case It'll look like this:

if ([NSVisualEffectView class])
{
    // OS X 10.10 code here.
}
else
{
    // OS X &lt; 10.10 code here.
}