We have been addressing this same issue for 2 months. We are using ArcServe for a backup solution but I beleive our issues are the same. Our conclusion:
The 2 servers CANNOT coexist. NT won't allow it. All service pack levels must be identical.
Before beginning, cut off access to your mail server so users can't alter your environement until your ready.
1) Since your new server is a BDC, you'll need to bring down your production server to start this process. We were able to install NT partially before installing networking to cut down on the down time for our production server. Alter the network properties of your production server to remove it from your domain. Bring it down.
2) Bring up your new server. Install NT from scratch, including service packs. Add it to your domain.
3) Install NT option pack for IIS configurations. (Production server is still down).
4) Install Exchange and SP for Exchange.
5) Install any backup agents onto your new server. ArcServe has an Exchange aware agent for live backups.
6) Once your new server is fully configured (with your production server down!), you can use your backup software to restore your Directory Store and your Information store to there original directory locations, but on the new server. In order for this restore to be clean you should rename the dsdata and mdbdata directories to dsdataold and mdbdataold and create new empty dsdata and mdbdata directories. Run your restore.
You should now have a new functioning server. If you do this work in stages you can keep your down time segmented to an hour or 2 each time. The largest window will be for your actual restore. We have not gotten a good handle on the time frame for this as yet. We are scheduled for a full test in the next 3 weeks.
