I will backup the DB just in case I need to go back to it...for sure.
As for VMs, how is it possible to upgrade hardware to 9 first when VMFS-3 doesn't support it? I'm going to have my ESXi 5.1 host accessing the same shared VMFS-3 datastores that my current ESX 4.1 host servers has access to. ESXi 5.1 host will also have access to VMFS-5 datastore which ESX 4.1 cannot have. I was thinking that I would use vMotion (Move VM Only) from ESX 4.1 to ESXi 5.1 keeping the same hardware version at 7 & VMFS-3 datastore since both ESX 4.1 & 5.1 support it. Then once it's on new host, do a Cold Migration of VM to VMFS-5 datastore, then upgrade VM hardware to 9.
What do you think?