As far the rollback is concerned, it is possible but going back to a 2.2 from a 4.0 / 4.1 Environment is not exactly easy but doable--You will need to do a full backup of up the Lotus\Domino directory--Just to be on the safe side, do a backup of Program Files\Research In Motion dir as well.
Reverting--> restore the data from the Data folder and the notes.ini. Re-install 2.2 version and bring the server back up online.
Re-install the Domino Version
1. How many user's do you currently have?
2. What version of Domino are you on right now? I am guessing 5x. You could go to 6.0 but i would suggest at least 6.5.5.
3. Question 1 is critical to find out the kind of power and processing you require for your server.
4. Use Windows 2003 Server with SP1
5. Review BES Documentation on IT Policies on what to implement
6. I would: Upgrade the current handhelds to at least 4.1 OS and then upgrade the server.
*I have been using bes since 2.1: Create a test environment (call rim / your rep for a trial server license) and build a copy of your environment and then perform the upgrade process. Review and test the policies along with the version of the handhelds that will and or will not work on your server pre 4.1 OS.
This may sound tedious but at the end of the day, worth it so you eliminate any anomalies that come up during the actual upgrade of the production server*
I am currently running 4.0 SP6 HF4 ( 2 Servers, 370 user's, IBM X Series Server with 4gb each with a remote sql server + attachment service), Windows 2003 Server and Domino 7.02 FP1. I also have a test 4.1 SP3 HF1 Environment, which is running smoothly--
Hope this makes sense..