elites can also upgrade into Z but only from Y. But seeing as I have never had a long-haul Y ticket (even after selecting full flexibility always end up B or M), I've never seen this in practice. But every time I call to upgrade, if O is unavailable I'm asked if I'd like to pay up to Y at which point I could use miles to book into Z. Never obliged. Would probably be cheaper to just pay up to Z.