You will need to be in line with the 2015 version at the first recert audit after it's released, which can be no more than three years after release.
I am sure that, in due time, the IAF will release a policy on the transition. But, while it might be a good idea for the certification transition to happen during a recertification audit, post release of the ISO 9001 5th Edition, in some cases, it might not be mandatory. For example, let's imagine that the revised standard official release date is September 15, 2015. If a certified organization has a certificate expiring September 16 or 30, they might not be ready for the transition audit so soon. Even the 3rd party auditors might not be qualified to audit against the new version of the standard so soon. Who knows, the accreditation bodies might invoke an accreditation upgrade of the registrars, before they can audit and certify organizations to ISO 9001:2015.
In those cases, I am sure the policy laid out by the IAF will allow the transitions to happen as part of surveillance, recertification and special audits.