Hi there @HaydnJ
In this case, I would recommend you explore the category combo override function
If your current data element/data is not disaggregated, this means the data will sit in the default category option combo. If you then apply the override, any NEW data will sit in the new category option combos. In this way, you will be able to view all of your historical data together when you do not select any disaggregations. If you start disaggregating your data in analysis however, you will not be able to display the historical data by these disaggregations.
I have an exercise that I use in training this concept, it is attached for your reference.
Metadata Creation Exercise.pdf (57.3 KB)
If you do end up making a new data element and applying a category combo to it, then the best way to link this data would be to make an indicator summing the old + new DE. That way you can still view the data over time; but this would not be my recommended approach given the existence of the above feature. Over time this gets really messy and you will just be adding more and more DEs to your system. By overriding the category combo, you are able to make changes using your already existing DEs.