with an s-on device,you are able to change some CIDs,but not to whatever you want. im not sure what restrictions there are,i did not explore it too much. i was able to change my stock cid of HTC__203 to HTC__001. superCID while s-on cannot be done without a certain htc-signed file present on the sd card.
i do STRONGLY reccomend you superCID your GSM evo 3d. there are different carrier ruus you can run,and also it makes your life easier while flashing PG86IMG files in hboot. if you retain a certain CID,the android info text document in a PG86IMG must contain that CID,or the flash will fail. i always superCID an international device.
set CID to 11111111(superCID):
if you dont have adb set up,download this: mini-adb.zip
it contains adb,fastboot and an md5sums utility
-extract it,place the unzipped folder on the root of your C drive
-open a cmd window(win 7,clock start button,type "command" or "cmd" in search box)
-change to mini-adb directory by typing,or copy/paste in the cmd window: cd c:\mini-adb you should see your promt change to c:\mini-adb>
-place phone in charge only mode,plug in(im assuming you have rooted via revolutinary and have drivers.if not use revolutioary drivers)
-in cmd,type or copy/paste the bold faced code: adb devices
-if you get your serial number back,boot to bootloader: adb reboot bootloader
-check for connectivity fastboot devices
-if you get a serial number back,check your cid first,in case you want to restore it later fastboot getvar cid or fastboot getvar all (will return other variables as well )