Hi,
You can achieve your task as listed below.
1. as you are doing,
ALTER TABLE tableName ALTER COLUMN [colName] (datatype)
provided that your data should be of required data type.
2. follow below steps
a. Add another column with required dataType.
b. import all data from old column to new column with dataType check.
c. delete / drop old column with the help of query / management studio once you imported all your required data.
d. Rename your new column as per requirement.
3. Recreate New Table with required table structure, import all required data, drop old table, rename new table as require.
Choice is yours.
Cheers