No reboot is required.
You have to make changes in all the hosts that you want the policy to be changed on.
- Round Robin (RR): Uses an automatic path selection rotating through all available paths, enabling the distribution of load across the configured paths.
- For Active/Passive storage arrays, only the paths to the active controller will be used in the Round Robin policy.
- For Active/Active storage arrays, all paths will be used in the Round Robin policy.
Note: This policy is not currently supported for Logical Units that are part of a Microsoft Cluster Service (MSCS) virtual machine.
Command will vary to your ESX/ESXi version, best is to refer the documentaion for exact command on changing the policy.
For example:
http://communities.vmware.com/thread/331346?start=0&tstart=0