Implement a bitrate detection mechanism in your application (bitrate = stream bits in buffer/ stream duration). Then, reset the decoder parameters, you may even need to reset the decoder with the new params.
These codecs doesn't support VBR by standatd design. Their bitstream doesn't hold any information about the bitrate. It passed outside from codec and its stream. Your responsability to tell the decoder new bitrate exacly with the first recieved packet with another bitrate. You can easily do it through USC interface by filling in->bitratefield with right one. Decoder checks that new bitrate differs from previouce one and explicity reinitializes itself.
Igor S. Belyakov