Upgrades are really depend upon what class you book. If you book sub or compact, you'll probably get a midsize. If you book mid, you might get full. If you book full, if you get any upgrade, it will probably be to a gas guzzler. It also depends a lot on the station. The larger stations (I've found) are harder for upgrades while smaller locations will be better. Overall, the upgrades I've found to be about 1/3 to 1/2 of the time... many times after asking for an upgrade.

As for Chairman's, you'll probably get more info from searching the forum. You have to be invited to that level. It's usually reserved for CEO's of large companies that have sole contracts with Avis for all of their employees. They did have an offer a year or so ago that a select few people could 'buy-in' to Chairman's for about 400. They didn't have that offer a 2nd time around.