Effect of Action is Modifiable by new Action:

Painful effects of past wrong action is reduced by
future right action.
Painful effects of past wrong action is enhanced by
more wrong action.
Pleasurable effects of past right action is reduced by new wrong action.
Pleasurable effects of
past right action is enhanced by more right action.
Behaviour (kamma) is almost always mixed:
Sometimes good, often bad!
Later effects are thus similarly mixed: Sometimes pleasure, often
pain...
Good begets good and dilutes/delays evil. Evil begets evil
and delays
good!

For details on the mechanics
of Kamma = Intentional Action see:
https://What-Buddha-Said.net/drops/II/Kamma_is_intention.htm
https://What-Buddha-Said.net/drops/II/Buddha_on_Kamma.htm
https://What-Buddha-Said.net/drops/III/Kamma_and_Fruit.htm
https://What-Buddha-Said.net/drops/III/Inevitable_Consequences.htm
https://What-Buddha-Said.net/drops/II/Good_Action_dilutes_Evil_Kamma.htm

Some
sound sources of the Buddha on Kamma:
AN 3.99 Lonaphala Sutta The Salt Crystal:
http://www.accesstoinsight.org/tipitaka/an/an03/an03.099.than.html