I assume you already ensured the hosts' hardware is supported by ESXi 5.1?
Basically you can start remediation of all the hosts at once and DRS will take care that enough hosts are available to run the current workloads/VMs (see Remediating Hosts). Without DRS in place, you need to migrate/vMotion the VM's manually. What you may do is to remediate one of the hosts manually to see whether this works as expected, before you start remediation of the other 5 hosts.
The last time I ran upgrades with detaching storage LUNs was on ESX version 3.0.
André