Hello you can not go directly to 5.5 you would have to upgrade to 4.x first then to 5.x or migrate the VM's off to a new host. I recently ran into this and you have to be aware of what versions of the VMware convertor tool work with what versions of ESXI. In my recent case they had ESXi 3.5 and we we installed a new ESXi 5.5 host. I was going to use a third party backup util from Veeam to move the VM's over to the new host since the VMware convertor tool did not support my ESXi and versions.
If you are sticking with the same host hardware then a 2 step upgrade would be needed if you are going to new hardware you may want to look at moving the VM's to the new host. **I only had 4 VM's I had to worry about moving.
Hope this helps!