I found a little hard to really understand your question, but I think I got it now.
The text boxes that were created when you dragged from the dataenvironment to the form, have some ‘data’ properties.
You need to create the desired combo box and set its ‘data’ properties (DataSource, DataMember, DataField) exactly the same as the text box.
Then you can delete the text box.
To see the ‘data’ properties you should select the ‘categorized’ view in the properties palette.
Please, tell us if this is what you needed.