You really only have one option with your controller:  backup to external media, remove drives, install new ones, configure RAID, restore. The SAS 5/iR:   
Poweredge server with SAS 5/iR Controller with two 300gb drives in RAID 1.  I need to expand the available space.  I haven't verified if the current drives are SAS or SATA drives yet. I would like replace existing drives with 2 x 2 TB SAS HDDs but I'm unsure of the easiest/safest way to copy over the data from the current array.  Ideally I'd like to be able to install the drives and copy over the data without having to reinstall the OS which is on it's own partition on the array.  That would mean swapping in one at a time, and copying data or rebuilding the RAID.  Since the drives are different I don't know if this is possible.  Will I need external duplication hardware/caddies to make this smooth?  
P Edge SC1430: Expanding existing RAID 1