Could you regain this info by adding a static prefix to the rolled id? So you know it was rolled, but not from which previous id. Where as new id's would have no prefix, so you can count new users as new.
Could just be the date of install in UTC as a prefix, the other part randomized on the first of the month... they could still calculate relative drop off, and still get better stats more anonymously.
tracker1|7 years ago