In this Ma’amar, the Rebbe explores the idea that Gevuros don’t begin to be corrected until Korach (!!) and Levi. Gevuros should really not be correctable (and then the evil is never transformed) but through Chesed, even the Gevuros are elevated and transformed.

Likutei sichos – Vol 30 p 103 ff G-dly Gevuros are kinder than kindness! Page 108.