Theres really no such thing as a W fare. Instead "W fares" work like UP fares. They're a coach class ticket with the cost of sitting in W included. Thus there's an underlying coach fare class. In order to purchase the fare you need both the coach fare class and W fare class to have availability . Whatever your underlying coach fare class is, is what needs to be high enough to allow upgrades using miles.

Basically Comfort+ is still an add-on like before. Just instead of selecting post purchase, the add-on is included in the fare you're paying.