Symptoms

Customer is currently using K2 API to assign workflow task to K2 service account which is a domain user then use logic and code to assign task to the actual users in your environment. They would like to change this to assigning the task to the actual users which could be domain user or external users (custom user manager).
 

Diagnoses

The documentation for custom user manager and updating roles have been provided.

 

Resolution

I suggested that you review the reference below regarding adding custom user manager into K2. If you need someone to help you set this up, I would recommend you contact K2 Remote Services department.
- http://help.k2.com/onlinehelp/k2blackpearl/DevRef/4.6.7/Creating_a_Custom_User_Manager.html

For running workflow instances, you mentioned that you will wait for the instances to be completed. New instances should be using the actual AD users or custom user manager once you have configured and redeploy your process.

We also walked through the process of creating Roles in order to be used in the process.
- http://help.k2.com/onlinehelp/k2blackpearl/UserGuide/4.6.8/Reference-WS_MCon-Roles-AddUser.html