Why can't I change the status of a column?

If you can't change the status of a column, it's because there are backlog items or tasks in that column. To change the status, first move all backlog items and tasks out of the column.

If you can't easily move the backlog items and tasks out of the column, here is what you can do: Create a temporary board column with the same status as the column that you want to change, move the backlog items and tasks to the temporary column, change the column status of the original column, move the backlog items and tasks back to that column (this will also change their status to match the new column status), and then delete the temporary column.