'Change Field Type' Button Missing When Converting Master-Detail Field to Lookup

The Change Field Type button is missing because one or more dependencies exist that prevent the Master-Detail relationship type from being changed. Identify which cause applies to your situation and resolve the relevant dependency first, then retry the field type change.

 

Cause 1: Roll-Up Summary Field Exists on the Parent Object

If the parent (master) object has an existing Roll-Up Summary field that aggregates data from the child object (for example, a Sum or Count field), Salesforce prevents the Master-Detail relationship from being changed.
Steps to resolve:

1. Navigate to the parent object's fields: Setup > Customize > [Parent Object Name] > Fields.
2. Locate and delete the Roll-Up Summary field(s) on the parent object.
3. Important: The deleted Roll-Up Summary field must also be permanently erased. Go to Setup > Customize > [Parent Object Name] > Fields, scroll to the Deleted Fields section at the bottom of the page, and click Erase next to the field to permanently delete it.
4. Once the Roll-Up Summary field is permanently erased, the Change Field Type button will reappear on the child object's Master-Detail field.

Cause 2: Cross-Object Workflow Field Update Exists

If there is a cross-object Workflow Field Update defined on the child object that updates a field on the parent object, Salesforce prevents the relationship type from being changed. Cross-object field updates are only supported on Master-Detail relationships and cannot work with Lookup relationships.
Steps to resolve:

1. Navigate to Setup > Create > Workflow & Approvals > Field Updates (or search for Field Updates in Setup).
2. Locate the cross-object Workflow Field Update that references the parent object and delete it.
3. Once the cross-object field update is deleted, the Change Field Type button will be available again on the Master-Detail field.

Additional Resources

Change the Custom Field Type
Notes on Changing Custom Field Types
Object Relationship Overview