can i change the prefered path policy from fixed(vmware) to roundrobin on equallogic luns while there are data on them and vms that are currently on? or do i have to empty the lun first and change it?
Changing the policy can be done online..
It can be done online, but if you want to be extra safe then I'd put it in Maintenace Mode first.
Equallogic also has a Multipathing Extension Module plugin that is recommended for configuring multipathing.
Check the VMware HCL for your specific model first to see if RR is allowed or check with Equallogic itself. But for an Active/Active SAN you should be able to set it to RR. Just make sure to change it on all your ESX hosts not just one.