Called Sage - they say they've not heard of but if we send our 2 GB of data they will try to re-convert. Doesn't help with an imminent go live. Only other observation is that the problem might be with Sales Orders that were DM and where the customer service dept entered the item as a negative to achieve a return.
Tried posting some current returns and everything shows up properly on the report so think we are going to just move forward and not worry about debugging further.