When you can't change the Agent of Record/assigned agent/manager on a case, it's usually a permissions issue.
Check to see that you (the the agency/brand manager) and the new agent both have the proper permissions enabled in the system.
How to View & Save System Permissions
To view, add, and save your permissions, go to My Account/Permissions.
If, based on the instructions below, you need to add a permission, check the desired box and click Save Permissions in the top-right of the Permissions tab.
Permissions the Agency Manager Needs
Edit Agent of Record
This permission is located in the Permissions tab, under My Account/Submission Options. Be sure you have this permission checked. If not, check it and save the permissions.
If you and your desired Agent of Record are in the same hierarchy group (this is the case most of the time), you need the permission to view/edit a descendant. Look in the Agency Management Display permission set on the right-hand side of the Permissions tab. Find the Hierarchy Related Permissions section. Make sure View Descendants and Edit Descendants are checked. If not, check them and save the permissions.
Multiple Brand Profiles
Changing the Agent of Record may require both users to have access to multiple brand profiles. If your agency only has one brand profile (i.e., there is only one possible brand all your consumers' policies could be under), you can skip this step. To give yourself (the agency manager) this permission, look in the Agency Management Display permission set on the right-hand side of the Permissions tab. Find the Branding Profiles section. Make sure Multiple Branding Profiles is checked. If not, check it and save the permissions.
Permissions the New Agent Needs
Multiple Brand Profiles
Case/Consumer Brand Access
Make sure your new Agent of Record is assigned to the brand profile of the relevant consumer/case. Go to Agency Management/Branding Assignments. Type your new Agent of Record's Insureio username into the Add a User box. Then type the desired brand profile name into the Add a Brand/Profile box. They will appear together beneath the search boxes, with the brand(s) listed above the agent(s) you have selected. Tick the box beneath the brand name to enable it for each agent you selected. Click the green Save button at the bottom of this section.