I received a reply from Trek today, and learned that the Alivio rear derailleur was in the original specs for the 2015 7.2; however, those specs were submitted prior to Trek learning that Shimano would be discontinuing the 7/8-speed Alivio rear derailleur (RD-M410). Thus, the switch to Acera is a "running spec change." If I understand it correctly (and I'm the first to admit that I may not), this means that when the Alivio stock runs out, the 7.2s will be built with Acera instead from that point on.