Sombra is dead. Chrysalis can't become good, because she needs to take love from others to live. Tirek will rot in Tartarus.
Conversion on Trixie worked because she was never evil to begin with. She was just too arrogant for her own good and got a cursed evil item.
Conversion on Discord worked, because again he's more ''I love to have fun and don't care about others'' than evil. Put in something to care for (Fluttershy) and bam he now take into consideration the well-being of others.