I wrote to the DOE a while back to find out what would happen to ds's statement as she reaches 16 this year, and received this reply.
I thought I'd share as it may affect some of you with older dcs 
Thank you for your email dated 06 March 2013 about SEN statements .
Education, Health and Care (EHC) plans will be introduced for children and young people who need them in schools, further education or training up to the age of 25.
We are currently talking to interested parties about how we should introduce the new system and how we should transfer children and young people with statements and Learning Difficulty Assessments (LDAs) on to EHC plans.
We want children and young people with SEN to benefit from the new system as soon as possible. However, the transfer to EHC plans will be a significant undertaking for local authorities and we want to make sure that the best service possible is maintained for children, young people and their families during this period. To ensure a smooth changeover, we anticipate that a phased approach to transfer will be necessary. Before we finalise arrangements, we will set out a proposed approach to transition for consultation later this year.
Until EHC plans are introduced, existing arrangements will remain in place for children and young people with statements and LDAs. For example, a young person with a statement moving into further education from school should receive a Learning Difficulty Assessment.
So basically, it hasn't been decided yet, and there will be a consultation!