I'm along the same line with . If this will be in-place upgrade, make sure to backup your VMs first, using free backup tool (VeeamZIP, for instance). The latest version of the said tool supports both ESX(i) versions. So, if something goes wrong with upgrade process, you will be able to restore previously backed up VMs to one host version (3.5) or to the other (5.5).
Cheers.