I have not had to do a reboot. I've changed many of our datastores to RR from the default VMware (Fixed) policy. I have seen I/O go active on all paths without a reboot.
Also you dont need to do this via command line it can be done in vCenter.
Select your host - configuration tab - storage - right click the datastore and select properties, you can change it to RR in 'manage paths'.
Remember that this policy is host based, meaning if you change it to RR on say DATASTORE1 on HOST1 if you look at DATASTORE1 on HOST2 it will not have RR unless you set it there as well.