Can't change column data type

Hi, i am using Sybase IQ 16.0

I have created a table, when I want to change one of the data type for a column for example from char to varchar, the list box is grey color and not allow me to change.

Right click table > Properties > Columns > Data Type

I have granted all permission for dba, but still not able to edit. (please see the attached file)

Any help?

Thanks.