The "transition costs" are mostly the actual reduction of revenue flow to the US Treasury, by whatever the amount of the FICA tax that is diverted to the private account system.
The upshot would be a SHORTER period before the net surplus of funds coming in from FICA is reduced to FICA income = Social Security outgo. Anytime after that point, of course, the general funds have to be taken from other purposes and paid out as SS benefits. Two things may be done then: either reduce the benefit paid, and expect the difference to be made up by the private portion of the account, or raise the general level of other taxation to make up the shortfall without deficit spending. Otherwise the borrowing against our future continues unabated. Of these choices, only one is fiscally responsible, and the reactionaries in Congress seem not able to make the connection.
Medicare, of course, will collapse much sooner than Social Security.
As a CPA, if I saw a company doing "off-balance sheet financing" by not recording all of its obligations, I would not give it a clean opinion. The government has had a humongous unrecorded liability and it's getting worse.