Why is my Change Field Type button missing on Master Detail fields?

Hi Manpreet
This is generally due to one of the following:

a) 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.

b) 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.

Hope this may help you:

Thanks