When you set the Classification rule builder it has two key setting which makes the impact on historical data
1. Rules overwrite any existing values &Rules overwrite only unset values
2. Lookback Window
If you are setting up the rule for the key value which has been classified through Upload the first condition you would need to check is if you have selected "Rules overwrite any existing values or Rules overwrite only unset values".
In selecting the first option "Rules overwrite any existing values" the value will be replaced with the rule builder only for the time you have selected in "Lookback Window" and onwards.
In the other setting if you have select "Rules overwrite only unset values" it will only look at the key value which does not have the classification data for selected Lookback window and onwards.
However lets assume you have key value which is being classified for past 6 months using Upload and you set the rule to overwrite the existing value for 3 months, in reporting you will see the key value with new value from last 3 months only and the data before that will have uploaded classification reference .