[G2 and Desire Z General] kusuriurikun:
No worries--we've all been newbs once :D

It IS in fact possible to downgrade then upgrade from stock 2.3.3 (I've actually done it; I've messed about with custom firmware back when I had a Samsung Omnia, but this was pretty much my "rooting deflowering" as far as Android phones went).

Here's how I did mine (this assumes you are running Windows and not MacOS X or Linux). I'm assuming you've backed up your apps first :D

a) Install Android SDK Tools (http://developer.android.com/sdk/index.html). I pretty much did the full install.

b) Install HTC Sync (I got it from http://www.htc.com/de/SupportViewNews.aspx?dl_id=1073&news_id=918 which is all in German but will still get you set up).

c) You will need to set paths correctly--in my case I had to click "Start", "Control Panel", "System", "Advanced", then click the button labeled "Environment Variables", then add in the "PATH" where the apps for ADB and (more important for our purposes) the platform-tools directory for ADB lives.

(There's actually a pretty good, in-depth guide at http://xdaforums.com/showthread.php?t=879701 on how to do this, but I'm just posting the Cliff's Notes version here :D)

d) Set USB Debugging on with the phone. Connect phone to computer using stock cable, click "Start", "Run", then "cmd" to open a command window. Type "adb shell cat /dev/msm_rotator" in that command window, hit return--if it gives some error about invalid length, your phone is visible to ADB and you SHOULD in theory be ready to downgrade.

e) Download and unzip the files attached in the article on downgrading (http://xdaforums.com/showthread.php?t=1178912) to the directory where "platform-tools" lives (usually something like C:\Program Files\Android\android-sdk-windows\platform-tools unless you installed it in a nonstandard location) and then in the command window type "cd C:\Program Files\Android\android-sdk-windows\platform-tools" (without quotes, and change that last bit to the actual directory if you installed it in a nonstandard location).

f) From there, follow the downgrade guide--if it hiccups, go on IRC to #G2Root and ask for help. (I didn't have any hiccups with this guide.) All the commands will be typed in that command window I mentioned. (Oh yes, you'll be getting intimately familiar with a command line and command window with this.)

g) Confirm (once the phone boots up after you re-flashed the ROM) that it is running 2.2.

h) Follow the guide on how to root a stock Froyo phone (http://xdaforums.com/wiki/index.php?title=HTC_Vision)--you won't have to worry about psneuter or any of that, because you've already done that (with fre3vo rather than psneuter).

i) Once you've confirmed it's rooted (according to that guide), then (assuming you're upgrading to 2.3.4 rooted or CM7 or whatnot) install the latest radio for the T-Mobile G2 (yes, you want to do this before ever messing with putting on a new ROM, otherwise you are going to find that stuff will go sideways). Get the latest T-Mobile G2 radio (http://www.mediafire.com/?vumdi3gges6ia4o), rename it to PC10IMG.zip, and follow this guide (http://xdaforums.com/showthread.php?t=970809&highlight=install+radio).

j) Install the particular rooted ROM you want to use. (I ended up just using a rooted version of stock in this thread: http://xdaforums.com/showthread.php?t=1202498).

And--barring a hiccup with the phone--voila, root. :D

This is pretty much what worked for me--can't guarantee it'll work for others. (Probably the scariest part to me was the radio update, but even this actually was relatively painless.)