Account manager targets are long-term goals that account managers set for themselves and their team members. They can be based on revenue, volume, margin, or any other metric that is relevant for the business. The team member hierarchy type determines the users that account managers can assign targets to. It can be either the manager hierarchy or the forecasts hierarchy, which are defined in the Roles and Forecasts Hierarchy pages in Setup, respectively. If the team member hierarchy type is changed in account manager targets, all the existing targets are made read-only, as a warning message indicates. This is to prevent any inconsistency or confusion in the target assignments and approvals. To edit the existing targets, account managers have to clone them and make the necessary changes. References: Choose Team Member Hierarchy for Account Manager Targets, Set Up and Configure Account Manager Targets, Set Up Targets for Account Managers Unit | Salesforce Trailhead, Assign an Account Manager Target