See https://docs.google.com/spreadsheets/d/17Pc1tIvqol6XJhuu97RlOfwy_Avbb9MEaRwhy2F529I/edit?gid=292544213#gid=292544213
This is a native amount fields paired with a date field that is set whenever the corresponding field is changed
Minor note: We can't calculate the otg change when the currency has changed, so we ignore any contributions that aren't in the currency of the most recent contribution (effectively, we treat each donor's history of giving on a per currency basis and this field only holds the change amount from the currency of their latest donation).