If we change the duration of any activity that will affect the downstream dependent activities & the overall prject schedule. The effect will depend on whether the activity is on critical path.
Softwares like MS Project & Primavera can be of help to forcast the effect this change on the overall project schedule.
To control this we can use crashing of the downstream activities to maintain the project schedule. Or adjusting leads & lags of the activitives can help in bringing the schedule inline with the planned one.