For a data element, I have a disaggregation of the following: children
Under Section 5 , Under 10 , Under 18 ,
with sample data as above. Now, what I want to add is a new disaggregation of Children < 3 months old.
Under Section 5 , Under 10 , Under 18  Under < 3 [ ]
How can I do that w/o losing the pervious data entered for pervious disaggregation ?
To do this you can always add more options to a category and update the corresponding category combination, this will not affect any data that was previously entered. Once you have added the new option go to data administration and in maintenance select the option to update category option combinations, then perform the maintenance. The data set should then show the new disaggregation.