The exact timeline for the new principal test (268) has not been decided, but chances are that it would not happen until December 2018 at the earliest with PASL starting in fall of 2019. Candidates currently in educator preparation programs will want to complete all requirements in a timely manner so that they continue to fall under the requirements of the 068 only. At some point the 068 will no longer be available, and then candidates will have to switch to the new requirements. We hope to have an implementation timeline available this fall.
I hope this helps!