There have been unofficial releases of the WM 6.1 ROM available for a while now, but I chose to wait until an official one was posted. While this "official ROM" has not graced the pages of Sprint.com or HTC.com yet (at the time of this posting), I felt comfortable with the claim that this is the official one - just leaked out a little early.
As with any ROM upgrade, you have to plan for a backup of your data, since the upgrade wipes out everything on the device. I use
Spb Backup on my Mogul, and the latest version has a ROM upgrade feature to it, which is a nice addition (I understand
Sprite Backup offers the same feature as well now). Previously, a backup from one of these programs would do you no good when upgrading a ROM because the file structure changes, and things wouldn't match up. I'm not savvy on the "behind-the-scenes" workings of these backup programs, but essentially this new ROM upgrade capability does some sort of magic to allow you to restore your data and have it work within the new ROM.
My main concern with backups is to make sure that my personal files, calendar, contacts and items such as that are restored. Apps can always be reinstalled, but it sure makes it easier to have that all restored so you don't have to mess with settings and whatnot!
Before doing my "pre-ROM upgrade backup", I removed some programs that I thought could be easily reinstalled without much effort. I also went through the My Documents folder of my main memory and moved anything that was exceptionally large over to my storage card. I did all this so the backup (and restore) would run a little faster. This is not a requirement, but a personal preference.
The ROM upgrade file for the Mogul is nearly 70MB, but it downloads pretty quickly from the mirror site I provided. Once available on the Sprint and HTC site, I'm sure it will download fairly quickly as well.
I connected to my PC via Active Sync and launched the .exe file. There's always that moment of nervousness when clicking "continue" on the desktop PC, knowing that the upgrade is about to begin and wipe out everything as you know it! Thoughts race through your mind: "Have I thought of everything?" "I sure hope I get all my files back!"
Despite those fleeting thoughts, the upgrade went without a hitch. It took about 11 minutes to complete.
Once the device finished the usual Sprint sofware install/reset, I was ready to perform the restore. The self-extracting backup file was nearly 80MB on my storage card (I wonder how large it would have been with the other items I had removed on there!).
Spb Backup detected that a different ROM was on the device, and automatically selected "ROM Upgrade Mode" for the restore.
NOTE: I have found that restores work best if you turn the phone off while doing it. I had a phone call come in during the middle of one restore and it messed things up, requiring me to hard reset and start over again. Theoretically, the program should shut down all services while it is doing it's thing, but it apparently does not stop a phone call from coming in!

The restore took about 5-10 minutes to complete (I wasn't watching that closely). After a soft reset, I checked everything out and all was well! The special ROM upgrade feature worked as advertised! Way to to, Spb!