An email I received from Avis about two weeks ago when I tried requesting a Van in the comments section of my reservation (emphasis mine):
The reservation for (location) has a request for a 12 passenger van, upgrades do not apply to the 12 passenger vans you have to reserve them and pay the rate on the vans, upgrades apply to midsize through luxury size cars based on availability.
Thank You
AVIS CHAIRMAN'S DESK
This seems to imply a new "downgrade" to the CHM benefits (At least the "market rate gas" is still around...)