Hello each other

We have been working with K2 for 1 year and are still building the first workflows.
Besides, we are still in a domain change from 2 old domains to a new domain. Here our problems with K2 start.
We have a workflow that assigns tasks to the users based on a group assignment. Now a user in this group is migrated to the new domain and loses all his old tasks. Only the new tasks are correctly assigned to the user again because I enter the new user into the group during the migration.
We work with form-based group assignments, not the role-based assignment of K2.
Now I would have to reset all running workflows in their activity for the new assignments so that the assignment is recreated again. This would cause all statuses in the affected workflows to be lost and reprocessed by the admin groups.
Now my question here before I reset all workflows: If I redirect the task to the new user in the management view, will all employees involved in the activity be excluded or will only one user be redirected?
I hope you understand my question and can help me somehow.

Stef