The Parol Evidence Rule answers the question whether or not a written contract can be amended orally at a later time.
The first point is that a written contract intended by the parties to be final and complete may not be contradicted by previous or contemporaneous written or oral evidence. In other words, one may not introduce as evidence prior drafts or discussions right up until an agreement is reached.
However, subsequent changes may be made by the parties to amend a prior written agreement. For example, you can enter into a written consulting agreement to render 20 hours a month of services. Later on, you can orally amend the agreement to increase or decrease the time spent.
In “real life”, if a dispute occurs, it may be difficult to prove that there was agreement with respect to the modifications.
Note that previous evidence can be used to prove fraud, mistake or duress.