Basically you don't need to convert the VM's in order to use them with ESXi 5.5 (see VMware KB: ESXi/ESX hosts and compatible virtual machine hardware versions list). Upgrading VMware Tools as well as the HW version should be fine.
For converting RDM's to virtual disks make sure that the size shown in the RDM's descriptor files (usually the initial LUN size) matches the real LUN sizes (in case the LUNs were resized in the past), otherwise you may end up with partly converted LUNs!
André