A big-bang PIM migration bets everything on one weekend. We prefer to run the old and new systems side by side and let the data tell us when it is safe to switch.
The reconciliation job
Every night, a job exports every product from both systems into a common shape and compares them field by field. The output is a short report: which SKUs differ, which fields, and by how much. During the first week it is long. By week four it is usually a handful of edge cases.
The rule
We cut over when the diff has been empty for seven consecutive days. Not before, and not on a date someone picked at kick-off. In practice this has always landed within a week of the original estimate — but when it has not, the business was glad not to have switched.
The cutover itself
Because both systems have been live in parallel, cutover is a configuration change: point the channels at the new system and keep the old one read-only for a month. Rolling back, if it were ever needed, is the same change in reverse.