Migrate In-flight data: Whenever a newer version of Snapshot is deployed in Process Server,
You have the following two option of what you want to do with the existing instances:
1) Move the instances to the newly deployed snapshot, this activity is termed as "migrate in-flight instance"
2) Leave the instances in the older version, so that newly deployed snapshot changes will not apply to these instances.
whereas Migration is the process of moving from a system to another.
In IBPM terms if you want to upgrade your IBPM version 7.5 to IBPM 8.6, one needs to perform the migration activity.