Well, when it appears in 'New DATs' if you double click it it should recognise it is an update to an existing DAT (if it is of course!) and ask you if you want to update. This is the way TOSEC DATs are since we only increase the date/version and the rest of the meta-data stays the same.
This is how most DATs should be (TOSEC and non-TOSEC), but if you gat a stubborn one that thinks it 'new' rather than 'updated', what you need to do is select 'Update' when given the 4 options of 'Update', 'Use default'(?), 'Use internal'(?) and 'Cancel'. Then select the existing DAT it should replace.
All existing settings should be imported, and the 'old' (now replaced) DAT removed.
Make sense?