Replacing SCSI drives in a RAID 5

I have a web server currently with three hard drives and a spare (each hard drive is 18.1GB). I am running out of space (only 900MB on C and 9 GB on D). I purchased 4 new drives, each 120GB and want to replace the old with the new. The drives are hot swappable. Everything i have read indicates that I can change out the hard drives one at a time and they will be rebuilt. However, although the new hard drives are each 120 GB, it sounds like they will be built identical to the 18.1GB config. Is this correct? If so, I read about rebuilding an array. What does that include? Will I lose my data? I am assuming I would have to take the server down to do this. Would this be correct?

Thanks, Jim