Does the crank itself need to be replaced? You could likely upgrade everything else (new bottom bracket, new chainrings) and just run the old crank with pretty much any setup you want. Unless there's something wrong with the crank arms, or you want to change the length or find a lighter model, etc, this should work fine.
My CAADX 105 came stock with an FSA double chainring crankset (BB30). I wanted to change to a 1x setup, so I removed the two stock chainrings, and mounted up a single 42T narrow/wide 5 bolt aftermarket chainring. Then I just swapped out everything else (rear derailleur, cassette, chain, shifters) with SRAM Force 1x products. It works great and the bike feels brand new.
The only thing that would give me pause with upgrading an old CAAD9 frame would be the limited tire clearance. If I recall correctly, it's 25 or 26m max clearance, and that aluminum frame is super stiff.