Change audio format on the fly?

[W3RCS]

Is it possible to change a connection from ulaw to adpcm without restarting asterisk? I'm looking for a command that would drop from ulaw to adpcm when my node switches from WiFi/ethernet to 3G CDMA. The node works beautifully on 3G with ulaw, but a minuscule drop in audio quality is worth the drop in bandwidth usage to provide more margin on the edge of coverage.