is there any way or i have to remove that and replace a new one at runtime?

WITH This
  FOR EACH loColumn IN This.columns FOXOBJECT
    loColumn.RemoveObject( [Text1] )
    loColumn.AddObject( [MyText1], [MyTextboxClass] )
    loColumn.MyText.Visible = .T.
  ENDFOR
ENDWITH

However, you can also do this at design time using the ADD OBJECT clause of the DEFINE CLASS command in the class definition of your custom column class.