Converting an existing system
Much of the work converting a system is very similar to setting it up. The big question is are you upgrading all the drives, or are you adding a similar drive to the existing one, and you want to make it a mirror. he approach is similar in both cases. You have read Setting up a (new) system?
Plan your new drive layout, install your new drives, and create the filesystems, mirrors, and lvms the same way as setting up a new system. The only real difference is that if you are only adding one new drive, you haven't got two drives to create your raid, so the command to create the array(s) is slightly different:
mdadm --create /dev/md/root --level=1 --raid-devices=2 /dev/sda3 missing
Note the use of "missing" in the place of a device to build the array on. This tells mdadm to reserve a place, but that you don't yet have a drive to put there, and it will create the array in degraded mode. obviously, once you've move your system across to the array, you will add the existing drive to get a fully functional array.