I was in exactly your position yesterday. So I did some research and I successfully upgraded from JF 1.5 to cyanogen 4.2.3
I found that the documentation wasn't quite as good as I would have hoped, but there was really only one piece that I had a problem with.
As for whether or not you HAVE to wipe, i'm not sure. But what I read said to wipe, and clearing things out makes for a nice clean install. I recommend wiping.
Steps I took to upgrade:
Downloaded cm-recovery-1.4.img (from what I understand, this is like the bootloader)
http://cyanogenmod.com/download/reco...covery-1.4.img
Downloaded update-cm-4.2.3-signed.zip (the new cyanogen rom)
http://www.cyanogenmod.com/download/...2.3-signed.zip
Downloaded signed-dream_devphone_userdebug-ota-14721.zip (the official 1.6 rom wich has the necessary google apps)
http://developer.htc.com/adp.html#s3
use a tool such as HashCalc to verify that the files downloaded without corruption
http://download.cnet.com/HashCalc/30...-10130770.html
Format your SD card (make sure it's NOT a "quick" format)
Copy the files to your SD card
Make sure your phone is fully charged before you start.
On your phone, run the terminal emulator. Elevate privilages by running:
su
(this makes following commands run as administrator)
The real magic is running:
flash_image recovery /sdcard/cm-recovery-1.4.img
(this actually flashes your new recovery firmware)
Now it is time to reboot the phone while holding the Home button.
This will bring you to the recovery firmware.
I highly recommend you first run the nandroid backup.
Then wipe the phone.
click "apply any .zip" and apply signed-dream_devphone_userdebug-ota-14721.zip
This is necessary because cyanogen's mod goes on top of the 1.6 firmware, which has the "google experience" apps.
DO NOT REBOOT YET
click "Apply any .zip" and apply the update-cm-4.2.3-signed.zip
when that is finished, reboot the phone using Home + Back.
Hopefully this all worked out for you.
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