It is possible to add new hardware modules into empty module slots in the relay. It is also possible to change an existing hardware module type to a different one. The rules for module slot allocation are the same as when configuring a new relay. Relays-Online can be used to determine the feasibility of the planned change. Changed/replaced hardware modules aren't refunded.
A spare hardware module alone cannot be used to modify a relay. The relay always requires a new license when its hardware or software is modified. The new license is a part of the Modification Sales delivery. Inserting an additional module into a relay or replacing a module type with another one without a proper license causes an internal fault in the relay.