Yesterday I argued that there is no wish on the continent to plan an orderly break up or slimming of Euro membership. Only a fast moving and unpleasant crisis, like that which hit the ERM, could force change in Euro membership. I also argued that there is currently no sign that the weaker members blame the Euro for their troubles or wish to leave.
I have before mentioned that to me the best way to handle any urgent need for a country to exit the Euro would be to move that country by unanimous vote from full membership of the Euro to candidate membership of the Euro under the provisions of the existing Treaty. These two categories already exist, and there is nothing in the Treaty to prevent a full member becoming a candidate member, though the Treaty was clearly written with a wish that the movement would all be the other way. Movement from candidate member to full member is determined by adherence to the qualifying criteria. As a country like Greece clearly does not meet the criteria by a very wide margin there could be a case for switching her the other way.
This would clearly need the consent of all. This is only likely to be forthcoming in a crisis of sufficient force to make member states believe that it is no longer tenable to keep a given country within the scheme. This could change the views of full members intending to stay as full members.
Why would the exit country or countries accept? The only way I could see that they could be persuaded to vote for their own loss of full membership is if they needed to receive financial grants or loans from the other members to pay their bills. A change of membership category could then be made a condition of the loan or grant, leading to their consent.