A subscriber identity is changed for a mobile terminal (10) through the initiation of a sending of an instruction to the mobile terminal (10), via a first mobile communication system (18, 30) to which the mobile terminal is attached, to change subscriber identity from a first subscriber identity (IMSI1) used in the first mobile communication system to a second subscriber identity (IMSI2) for use in a second mobile communication system, and the initiation, after obtaining knowledge of the mobile terminal having attached to the second mobile communication system based on the second subscriber identity, a detachment procedure in the first mobile communication system in relation to the first subscriber identity. The mobile terminal (10) receives the instruction to change, attaches to the second mobile communication system based on the second subscriber identity and detach from the first mobile communication system after having attached to the second mobile communication system.