According to the FAQs regarding IATA "Cabin OK":
IATA is a standard-setting organization. Is this new carry-on size a new standard for the airline industry?
No. Those who participate are not being asked to change their existing requirements. Some or all may continue to accept bags larger than those qualifying for the IATA Cabin OK logo.
Actually, the whole FAQs are talking about none enforcing this policy. Another comforting part:
My current carry-on is larger than IATA Cabin OK bags. Will airlines now insist that I check it? Will I have to buy a new, smaller carry-on?
No. Each airline is free to set its own policy regarding baggage, but this new initiative is not expected to result in any sudden spate of baggage rule changes.
It seems that the whole idea behind the "Cabin OK" program is to treat "Cabin OK" bags as acceptable. Each airline still has the full right to determine if they want to follow the "Cabin OK" specifications, but I do not think they will do this anytime soon. In the future things might be different and we might be looking at the future specifications of cabin luggage. How far in the future no one knows yet.