You need to check this checkbox **`Child records can be reparented to other parent records after they are created`** (labelled as *Allow Reparenting*)on the field so that their parent can be changed (For Example, from Community A to Community B).

Steps to achieve this:

 1. Go to the detail page of the "Baseline Survey" object and find the master-detail relationship field.
 2. Edit this field and select the above mentioned checkbox.
 3. Save the changes, now you should be able to change the master (Community) of the detail (Baseline Survey) records.