I just bought a brand new T+ flight pass and they do indeed book into M. They are honouring the certificates (as previously mentioned) and these T+ segments are upgradeable with SWU/NAU until Feb 28.

Before I shelled out the $3k, I called to double check all these details and the agents seemed very aware of the "glitch" and were willing to help passengers upgrade current T+ bookings in M/U without restriction (i.e. no 4 day window applies).

Hope this helps anyone else thinking about pursuing the T+ flight pass option to take advantage of the booking class realignment glitch.