Too early to tell. From my experience with data integrity issues first make the numbers manageable by linking data to another database (cross checking). If possible do a mass update then and what remains a red flag handle manually. This is a simplified version though, can be much more complicated. If some data is not in a database they can connect to or even worse is on paper, that can make it very labor intensive.

I have regularly coordinated and fixed data integrity issues myself. Worked for a large company that was often audited.