Now it may seem as if I am going to contradict myself but here goes.
Karma that we have committed before, if unpurified will certainly manifest when the conditions are right.
But if can somehow purify our karma before the karma starts to manifest then we can certainly change or lessen the karma. Dont we want that. Purification practices will be effective with the four opponent powers in place.
Below is one kind of purification practices there are many others too .... http://blog.tsemtulku.com/tsem-tulku-rinpoche/prayers-and-sadhanas/sadhana-prayers.html#13