Your old system stays in charge until you say otherwise.
Nothing is imported until you have seen what will happen. Every import is a dry run first, reporting row by row what will land and what will fail and why — you approve an outcome, not an intention.
A mirror keeps reading your existing system while the new one fills up, so there is no cutover weekend. It is explicit about which side is the system of record, and conflicts are shown with both sides rather than resolved quietly.
And before any of it, we replay your last ninety days through the system and show you what would have been automatic — counted from your own history, with what we refused to count listed underneath.