§1. For a cleric already incardinated to be incardinated validly in another particular church, he must obtain from the diocesan bishop a letter of excardination signed by the same bishop and a letter of incardination from the diocesan bishop of the particular church in which he desires to be incardinated signed by that bishop.
§2. Excardination thus granted does not take effect unless incardination in another particular church has been obtained.
Source: Code of Canon Law (Vatican.va)