Yeah, I got into it big time back then. I even got a CE 4.2 app to work on my Jornada 820, and a bunch of MIPS Pocket PC apps working even on my CE 2.01 Palm-size PCs - though there seemed to be bigger incompatibilities between the Pocket PC 2000 coredll.dll and the CE 2.11 Palm-size PC coredll.dll than between the Pocket PC coredll and the H/PC Pro one. Several apps that worked in H/PC Pro after hexediting the OS version wouldn't work on CE 2.11 Palm-size PCs.
Generally, the fewer DLLs required, the better the chances of making an app work in 2.11. Fewer DLLs generally means fewer function calls, which generally translates into fewer chances of incompatibility. And, of course, you have to change the OS versions on all the DLLs too.
