Regarding Dell Equallogic storage arrays, is it possible to replace the active controller module despite the fact that it's processing volume I/O? If not, what should I do before replacing it?
You move no LUN.
The way it work is the passive will activate and the active become passive. The IP is the same, so the gear connect to the new active, the problem you can face is a bad failover in case of a bad configuration, thus your hypervisor can lost access to the SAN temporaly. Its the problem you can face.
To switch to the passive controller manually the easiest way is to unplug the rj45 from the active controller, the passive (still plugged) will take the load from there.
Its actually a good test before putting a equalogic into production.
To move LUN its not when the active/passive controler do balance. Its when you have a failover equalogic SAN configuration. When the active SAN fail, the passive SAN automaticly take the load, but when the failed SAN is ready to rebecome online you need to redirect to a new iscsi target to retarget the correct active SAN. As you can see its not your case.
Before replacing the active controller, you should perform a "restart" from Group Manager. The process this starts makes an attempt at a non-disruptive failover, and provides a quicker handoff of iSCSI sessions than simply removing the active controller would. Whether or not this causes an outage for you would depend on whether your host configuration follows Dell's best practices (e.g. iSCSI login timeout of 60 seconds).