If upgrading from previously installed mod I like to leave the old mod in for performance comparison. For example when upgrading my dirty drives, I had a very good roll grade 4 mod installed. I rolled two grade 5 that were poor. The loss of the secondary effects from the grade 4 for the very minor increase to speed was not worth the swap at the time. So I held onto the grade 4 for a while longer. Hopes that helps.