If you are using the same Exchange store, you should be ok.
Basically, you have two choices.
- in place upgrade 4.04 to 4.2, install new 4.2 server (exactly the same is the old 4.2 server with the exepction of OS)
OR
- install new server as 4.04. DIRT Export old and DIRT import to new server. Run inplace upgrade to 4.2.
basically DIRT does not care about drive letters. it wants to know where the SQL database is and the Exchange store. The files are basically in PST format, and DB files. The DIRT script just helps out importing/export and a few other tools.
Check out the video. It's great!
http://www.ciscounitytools.com/App_DisasterRecoveryTools.htm