I don't konw the details, but I think there is some provision for younger people to recoup what they've already paid in, or at least get benefits based on that amount without their other private investment income interfering with it. But I think the plan is still under development.
I don't know all the specifics either, but from what I understand, you will only be allowed to invest a portion of yor SS.
I'd be willing to take a quarter of MY contributions not counting my employers as a payout to put it in the new system. For me that would be a pretty penny in thirty or so years.