The Diamond Courtesy Upgrades are a long haul thing only, they don't give them on short haul (those would be op-ups). You are right they are rare, I have received my first one just last month to C on SFO route, having heard about this for years and not believing it. Most of the times the business cabins go out already booked or with a lot of upgrades. As mentioned upthread, doing a mid week to ORD in February or similar would be your vesy best chances of experiencing the elusive CU to business.