How to convert a varchar column type to date type without losing the dates

You will need to adapt this based your your exact table structure but something like;

CREATE TABLE temp (startdate varchar(255), stuff varchar(255));

INSERT INTO temp
SELECT startdate,stuff
FROM mytable;

TRUNCATE TABLE mytable;

ALTER TABLE mytable ALTER COLUMN startdate DATETIME NOT NULL;

INSERT INTO mytable
SELECT CAST(startdate AS DATETIME), stuff FROM temp;

DROP TABLE temp;

First, create the new column with type data Next, run update query, to populate the new column with the value of the old one, applying any conversion if needed Next, drop the old column Finally, rename the new column to the old one