I have an HTC Evo 4G for Sprint, running Gingerbread. I would love to be able to get this dinosaur onto Cyanogenmod, if possible. However, I have no experience with phones, and it seems that none of the methods that I am finding are working with my specific phone (every one that I've tried has a hitch somewhere). If anyone with more experience in the field could weigh in, I would really appreciate it. Maybe it is just a lost cause, I honestly don't know.
Unfortunately, CM development stopped for the Evo 4G at CM 7.2, which is based on Android 2.3.7, which is still Gingerbread. However:
http://wiki.cyanogenmod.org/w/Install_CM_for_supersonic
If that S-Off tool doesn't work, you can try doing it through HTC's dev site. That's how I unlocked my HTC One M8, and I don't think voiding your warranty means much given the phone's age.
http://www.htcdev.com/bootloader