In Xcode, how can I utilize new UI APIs for Yosemite, but then revert to older ones for older OS'? I have been looking all over the place and can't seem to find an answer. (I'm probably not using the correct terminology) 

I'm building a simple AppleScript ObjC program and I want to use the new UI API for Yosemite (NSVisualEffectView) but, as expected, it crashes when I open it on Mavericks. How could I get it to use old APIs instead without having to create a completely different app?
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.
}