Replacing certain status bar icons are very easy if you know where to look and what to do:
Find SystemUI.apk in "/system/app/" on your phone and send that to your computer.
MAKE A BACKUP just in case you have any regrets later on.
Change the file extension to .zip instead, and open the zip file.
Navigate to "/res/drawable-xhdpi/" and look around for the icon you want to replace.
If you're lucky, the ROM you have will have all the different carriers' icons in that same folder. In that case, just copy the icon you want and rename it so that the filename matches the icon you're trying to replace, and overwrite the icon to be replaced.
Rename the .zip file back to .apk
Optional: Use zipalign to optimize the modified apk
Overwrite the SystemUI.apk on your phone with your root file manager of choice. You might get a lot of FC dialogs. Ignore them and hold down the power button to reboot.
Once you're rebooted, changes should take effect.
If your ROM doesn't have the stock icon you wanted I can post that later. If you don't have zipalign I can also post that, or you can download the Android SDK from Google, which includes many useful utilities.