user: Question: What was the reason for change in staff in formal installations on March 15, 1865?
Columns: ['State (class)', 'Vacator', 'Reason for change', 'Successor', 'Date of successors formal installation']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['Maryland (3)', 'Vacant', 'Sen. Thomas H. Hicks died during previous congress', 'John Creswell (UU)', 'March 9, 1865']
SQL:

assistant: SELECT "Reason for change" FROM "Table" WHERE "Date of successors formal installation" = 'March 15, 1865';