Update VSphere 5.1 to VSphere Hypervisor 5.5?

I installed VMware VSphere 5.1 free trial, and created several virtual machines.

I mistakenly thought I downloaded and installed VMware VSphere Hypervisor 5.1.

Now the free trial has expired.

I would like to "upgrade" the VMware VSphere 5.1 with expired license to VMware VSphere Hypervisor 5.5 and still preserve the installed virtual machines.

Can this be done with the VMware VSphere Hypervisor 5.5 interactive installer?

Or must I do a new install?

If I must do a new install, can I preserve the existing virtual machines during the install?

BTW, I manually backed up the VMs just in case.

Thanks, Mike