Missing 'Change Field Type' button on Master-Detail Fields

1. The master object has an existing Roll Up Summary field for data on the child object. In this case, it is necessary to delete the Roll-Up Summary field on the parent object to permit the Master-detail field type to be changed.

Note: Deleted Roll-up fields should also be permanently deleted. 

Go to Setup | Customize | Name of the Object | Fields | Look for 'Deleted Fields' at the bottom of the page and open it | Click 'Erase' on the fields you want to permanently delete.
 

2. There is a cross object Workflow Field Update defined for the child object to update a record on the parent. Cross object field updates only work on master-detail relationship. In this case, it is necessary to delete the cross-object field update to permit the Master-detail field type to be changed.