I believe you will find the column you are trying to change has a formula that is setting it's value. To be able to change this value, you will need to go to the Column Info for the column, click on the formula column property, and then click on remove. This will not remove the values from the column, but will just remove the formula. The values in the column have now become just regular numeric values. Now you will be able to change the data type to character.
Jim