Controllers with different capacities within the same series (M-series or S-series) and controllers of the same capacities within the same series, can be migrated online. For example, the lower capacity MD Plus controller can be migrated online to the higher capacity MX controller and the MD Plus controller can be migrated online to the MQ controller which is at the same capacity as the MD Plus.
It is highly recommended that you migrate the second controller in a redundant pair shortly after migrating the first controller. That is, do not migrate one controller in a redundant pair and leave the other controller to migrate at another time.