From what I have read, they are not going to be retiring any cars, whether M2, 4, or 6, until the fleet is built up to a minimum size. Once they reach a certain threshhold of seats is when they start retiring. Since CT seems intent on getting the max order, 342, I think, (not 380; 342 was authorized in the original legislation, although the order allowed for 380 but add'l legislation would be required - look back in the thread for a link to the CDOT memo I posted), CDOT and NY could conceivably retire some of those smaller fleets of M4's and 6's.
I would think, though, they'd keep a reserve, similar to the M3a reserve (there was supposed to be an M1a reserve for Yankee service, but they decided it wasn't worth it). These cars can also be used across EOH divisions. That was also a plan for the 4400 washboards, but as Noel has posted on here, they stored them badly and did not maintain them.
Next stop, Willoughby
~el Jefe ("Jeff Smith Rules") :: RAILROAD.NET Site Administrator/Co-Owner