There is no confirmed plans to totally switch over to digital radio at this time.
But, if a total switch to digital radio ever was made then existing car radios would no longer work and would need to be replaced or a separate tuner unit would be required to be plugged into any input on the current stereos.
Speculation is that the switchover date may be 2015 - but this could change.
Well before then car manufacturesrs will have introduced digital eqipment. But as Chuck says all current analogue radios will become defunct at changeover.