Audio Codec

Hi, As it is very clear that any End Point may operate in specified set of capabilities depending on the policies or bit rate or whatever. When the call is in proceed on a particular codec, what are the factors which decide on changing to a new codec ? Is user allowed to change codec when the call is in proceed ? ( Assuming that the other end point indicated MaxPendingReplacementFor capability ) Can anyone help me in this regard ? thanks Martin D.