or it may do harm to our machine

Note: you may want to make a backup in case something goes wrong (I recommend Titanium Backup). I will not responsible if something goes wrong.

1. Download the 2.3.20 SBF file from the link above. Note, this is Froyo.
2. Download RSD Lite (the latest is 4.9 and it doesn’t need an INI patch but you may have to select “Config->DeviceId->First Come First Serve)
3. Connect the phone to the PC first, then switch it off (if you connect it while it’s off, it will turn itself back on)
4. While you hold the Cursor Up button on the slide-out keyboard (the triangle that’s closest to the Enter key), turn the phone on. This gets you into boot loader mode. You should see “BootLoader D2.37, Battery OK, OK to Program, Data mode USB”. This is the only special boot-up mode for me, booting with volume-up gave an error in RSD Lite.
5. Using the “…” button in RSD Lite, select the SBF file you downloaded in step 1. If the SBF file was in a ZIP file, you have to unzip the ZIP file first.
6. Click the Start button in RSD Lite. The message on the phone screen changes to something like “Upgrade in progress” and you’ll see RSD Lite go through several steps. If something goes wrong, the phone won’t start up but you can start over from step 3.
7. RSD Lite will reboot the phone and will then tell you you have to manually power on the phone. I think this is a mistake: I think it means power OFF the phone. On one of my phones, RSD Lite said it was finished after I powered the phone off, on the other one RSD got stuck on that message and gave me a big “don’t do this” when I closed the program, but the phone worked fine afterwards.
8. Your phone will now boot the stock Froyo distro but you should still have all your programs and settings. You will also see the Roaming logo (blue triangle), call *22899 (*ACT99) to re-activate the phone.
9. Tap the Menu button, then Settings, then About Phone (at the bottom). Then tap System Updates. It will tell you there’s an update and it will ask if you want to download it. Tell it to go ahead. Once the (106MB) download is complete, tell it to install it. It will reboot and update the OS, and this time it should succeed where previously it would fail. The phone will reboot once again, this time with a red Motorola logo: that means you got Gingerbread.
10. Google for Motorola One Click Root for Droid 2 and root your phone again.

Z4Root from the instructions above didn’t work for me: it keeps looping while it’s trying to trigger the exploit on which it’s based but evidently that exploit has been fixed because it eventually just locked up my phone.

There’s no need to do any recovery or anything and you don’t need the “monster” zip file above. You don’t even need to remove your battery or your SD card. And you don’t need the D2 bootstrapper.

Thanks to all the people who provided information here and elsewhere!

===Jac