Very easy, but make sure everything is backed-up.
I tend to ensure each SATA cable goes back to the correct drive and plugged back into the correct SATA port and take your time, if your overclocked save the profile to the BIOS or write down the settings just in case.
I have found that it never seems to go as smoothly as you would like but as RJC says try to make sure each SATA cable goes back to the correct drive and plugged back into the correct SATA port and take your time. Definitely take your time.
Check everything twice before committing yourself.