I think upgrades EWR-LAS are a total crap shoot (pun intended). I've seen flights sold out in F a week in advance, and I've seen Gold companion upgrades be processed (and having shelled out of pocket for the full fare first class ticket, I was might peeved...).
With all the convention activity in LAS these days, there can be an influx of business travelers, many of whom can fly paid F on this route (>5 hours, which I believe is a common cutoff for companies). So the timing of a convention can have a big impact on F availability.

The other thing worth pointing out -- the difference between Y and Z (the discounted First class ticket) is only $100 r/t, and besides better protection against involuntary downgrades, you get the 50% class of service bonus miles, or about 2,200 r/t, or 4.5c/mile, not so bad. The "A" fare, full fare first class, is $200 more than Y, so the COS bonus isn't really worthwhile. The only issue with Z vs. A will be availability, but if you find Z inventory and can book it, and don't plan on changing your flight, it would be fine. And if you do need to change your flight, and there is no Z inventory, you can still always buy up to A for $50 o/w.