Before you could only upgrade a few booking class, typically full fare or almost. Now you can upgrade any booking class but it costs more miles. One exception, I think it was noted in their terms and conditions, Z and I cannot be upgrade to first class, as well as award tickets cannot be upgraded with miles either.
You can also redeem miles to book "any seat" wlthough typically full fare booking classes (PJWY) can sometimes not be bought with miles it seems. However, they have never promised that you can upgrade any fare at any time, capacity control is still regulated for upgrades like it used to be since you need to hve F/O/A availability.