I build and rebuild these all the time. If you have a good backup of the system, and the software CD's and boot floppies, just follow the instructions under installing software. You must have the same make and model drive, you can't just install any drive. The boot sectors are different on different drives and UNIX is not too flexible. It is also best to do a low level format if the drive isn't new, you can find those tools on the drive MFG's web site.
One error in the doc's is when it comes to the point where you type "/autoinstall" That is wrong! It is "/etc/autoinstall" -minus the quotes of course.
If you want to protect yourself, get 2 drives and load them both identically and install the extra in the cabinet "UNCONNECTED" beneath the main drive, then you can do a powerdown, change the connectors, reboot and restore from tape.
Load time for 1 drive is about 2 hours, a restore takes 30 minutes to 1.5 hrs depending on the number of users.