I did not ask Celebrity when this started, but there number is posted on their website. I spoke with Keith, a Celebrity Cruise Planner, at the Celebrity number.
Upgrades ARE available to Captains Clubs members, and I was upgraded from a C2 to a C1 on Concierge Class for an upcoming cruise on the Equinox in 2009. I can only tell you what I was told. I asked when I would be eligible for "upgrades" and Keith stated after our 5th cruise we would. He also mentined that upgrades were not available on the Aqua Class.
There are many things which aren't "posted" or "in writing" but they are still available.