First of all, no problem moving VMs from one ESX to another if they use different multipathing policies.
Second, if you are using IP hash, this means you are using etherchannel (or LACP) on the physical layer - configuring/unconfiguring this almost always require that the uplinks are not being used by any VMs, so you have time to sync the physical and the virtual configuration. It is not a downtime, but you need to plan it accordingly - all depends on your physical network capacity.