In case that you need to plan the change in BoM for future, it is necessary to use Change records.
Create Change record
on the next screen fill Change number desription, Valid from, Reason for change and Change no. status field:
tick check boxes as shown and press „Save“:
In our example (material 64103) the component 39801 will be replaced by component 39802 with Validity from 25.10.2015.
change component number here and save the BoM:
Now, if you reopen the BoM: