Is it possible to migrate vm's for a host running esx 5.1 to a host running 5.5?
Yes, It is fully supported.
Use unregister and reregister if it is a shared SAN environmaent
Use a method of export and import (ovf) if it is isolated ESXi
Venkat
Award me points if it answers your question.
Yes, it is possible. Do you have the two hosts running under a vCenter?
Yes it is possible. The methods will vary depending on how your environment is set up.
Just keep in mind that if the VM's are going to move back to the 5.1 hosts not to update the VMtools/Hardware version while they are on 5.5
You can use free tool called Quick Migration that is included in Veeam Free edition. With Veeam Quick Migration you can promptly migrate one or more VM between ESX(i) hosts, datastores or both. With the latest product version you will be able to migrate VM from 5.1 to 5.5.
Cheers.