As per the comments in that article (and confirmed by Ivan's correspondence -- with PNR's! -- with RJRC CS) only the fare buckets mentioned in the mileage earning tables for the airline in question are eligible for either mileage or segments calculation (as well as "R" mentioned as earning only quarter of miles/half a point).

I have a slew of Y flights not eligible (three out of four different buckets present in the assortment are ineligible...), and there is the cryptically worded mention about R "economy" fares (there are also business fares with that letter about), so I sent an enquiry about them to RJRC. But if the answers are what I expect, I'll be most probably crediting all the OW flights before 1st of April to MH and then see whether BA(C) credits also pre-change booked alliance flights similarly to BA/AA/IB ones, to correspond with the old system in terms of tier point amount. In that case I'd already have Silver renewed with future flights. Otherwise I'll continue crediting to MH.