On the recently closed CAD proposal it says:
Keep the proposal alive by restarting it. See Closing a Proposal After 1+ Year in Definition.
On the linked topic it says:
A New Beginning for Your Proposal
Closing a slow-growing proposal doesn't have to be the end of the process. If you still believe your site will work, feel free to restart the proposal and prove it.
In April of 2014, somebody asked about this process on the meta there, but the answer remains secret.
How do you restart the proposal? Is there a restart button somewhere? Is there a "keep current followers, questions, and topics, and extend proposal another year" procedure? Do you just start from scratch, maybe post a signpost topic on the old one, and see if the followers read it and repost their example questions?