(redirected) poweredge 2950 change all drives while maintaining OS

I have a poweredge 2950 with 5 x 145GB sata HDD's and a windows 2008 R2 installed, and I have 6 x 2 TB SAS hard drives in which I bought to upgrade all hard disks on this server, how can I do that without losing OS ? I am new to the environment and I would appreciate giving me tips and steps on the matter, Thanks a lot.