Welcome to the Community,
both approaches will work. However, since unmounting a datastore has to be done on each host individually, I'd suggest you simply delete the datastores, rescan the hosts (unless this task runs automatically) then unpresent the LUNs on the storage side and rescan again. After that you can delete the LUNs on the storage, create new ones and present them to the hosts.
Unmounting/detaching is only required if you want/need to preserve the VMFS datastore on the LUNs.
André