Can the access control be changed without stopping the factory?

The order that works: first the new head end goes up in parallel and the clean list is loaded, because a migration is the only chance to throw out duplicates, credentials with no owner and people who have left —once it is in it is history and nobody touches it. Then the doors are moved over in groups, starting with the lowest-traffic ones and leaving the shift entrances and the critical ones until last. Each group is moved outside that area's hours, tested in both directions and left running a couple of days before touching the next one.

What has to be ready before starting: a roll-back plan door by door, credentials that work in both systems while they run side by side or a staggered card handout with its collection point, and who opens by hand if a door is left half done at six in the morning. It is the part of the project that gets written down least and the one that decides whether the change is remembered as one bad weekend or as two months of incidents.