How to handle active tickets while migrating from one ServiceNow instance to another

[SantanuKumar Sa]

We are migrating from one ServiceNow instance to another. There are active integrations in both the instances. How can we handle the open tickets in instance A which are still active at the time of migration. 

[Mark Manders]

A lot can go wrong with active tickets. End Users can have received notifications on their tickets from the old system and will reply to them (or use the link to update their tickets). Moving the tickets can cause issues when there are (work)flows running on them, or maybe logic is different (mandatory fields on the new instance, not available in records on the old one).

Is there a way to close the old instance for new tickets and start working with the new one, so you can finish as much of the old ones as possible and maybe create new ones on the new instance?
Tickets that are in integrations can also become tricky, although correct correlation ID's could make it easier if you copy those from old to new (so the other party will receive the correlation from the new instance and will integrate further on that), but that also depends on how the integrations are set up.

Logic like SLA's also need to be taken into account (depending on the kind of tickets we're talking about). The easiest way (if possible) is to close the old system for new tickets so you can just handle the open ones as much as possible and create new ones. This of course also depends on the numbers we are talking about. 5000 changes open with possible end dates in Q4 will need to be handled differently than a request to reset a password.