Reverse engineering: Activity to create an easier to understand and more abstract form of the system.

Forward engineering: The traditional process of moving from a high$-$level abstraction and logical$,$ implementation$-$independent design to the physical implementation of the system.

Two major dimensions of alteration are:

$1-$Change in functionality

$2-$Change in implementation technique

A change in functionality

$-$Comes from a change in the business rules.

$-$A modification of the business rules results in modifications of the system.

$-$Change of functionality does not affect how the system is implemented, that is$,$ how forward engineering is carried out.

$-$Modifications made to the system that affect its intended behaviour.

A change of implementation technique

$-$Refers to modifications made to the system that affect how it is built & structured, without affecting its intended behaviour.

$-$Changing the programming language from procedural to OO language.

$-$An end$-$user of a system never knows if the system is implemented in an OO or a procedural language.

Change in functionality is not clear to me$.$ Please give examples. Thanks