Hi,
You must of used MC_STYLE_ENABLED property of CL_GUI_ALV_GRID class to make fields enable right? only for new entry just apply this property for key fields also. and in subsequent changes for the same entry just make sure that you use property MC_STYLE_DISABLED of the same class.
refer to the program BCALV_GRID_EDIT and the related programs in the same name series.
thanks,
Aswath.