As I understand it, those with an IMC rating attached to a JAR licence before April 2012 will continue to be able to exercise the privileges as before, but in addition the IMCr hours will count towards the new competence based modular IR. To my mind, that's much better situation that we have today; i.e. those who have a current IMCr and only want an IMCr will continue to be able to use it (probably called a restricted UK IR), and those who want a full IR will be able to use the IMCr as credit within the new modular system.
Hurrah, this is what I've been wanting to know. Thank you sir.