When you remove the datacenter (from the 4.1 vCenter) in step #1 you will be disabling HA on the hosts so you won't have HA protection until you add the hosts to the 5.1 vCenter and HA gets configured on them. If you want to limit the amount of time you don't have HA protection you could add the hosts to the 5.1 vCenter first and only remove them from the 4.1 vCenter afterwards.
Elisha