If I may ask, how is one appointed to be a bishop? Is this a life-long, irrevocable appointment?
Bishops are appointed to particular dioceses by the pope. The appointment is not necessarily lifelong: a bishop can retire or can be transferred to another diocese by the pope. However, a retired bishop does not cease being a bishop; episcopal ordination is indeed permanent, as is priestly ordination. Retirement simply means that a bishop is relieved of his diocesan duties, not of his episcopal consecration.