Changing the value to negative
In my opinion, the most elegant solution would be to leave the original values and to define how you operate with them (adding, substracting, etc.) at the Aggregator Plus level, but I am not familiar with that module, and I don't know how easy it would be to extend it.

If you want to modify the value of these fields before saving the node, there are at least two possibilities I can think of:


Using Rules (it involves creating some custom PHP code). The following example (using increment instead of changing the sign) can give you a hint: https://www.drupal.org/node/1389074
Creating a custom module and implementing hook_node_presave(), in order to modify the fields before they are written to the database.


Hope that helps,