Work on the Offline Framework for the Pierre Auger Observatory was
started in 2003 to create a universal framework for event
reconstruction and simulation. The development and installation of
the AugerPrime upgrade of the Pierre Auger Observatory require an
update of the Offline Framework to handle the additional detector
components and the upgraded Surface Detector Electronics.

The design of the Offline Framework proved to be sufficiently
flexible to accommodate the changes needed to be able to handle the
AugerPrime detector. This flexibility has been a goal since the
development of the code started. The framework separates data
structures from processing modules. The detector components map
directly onto data structures. It was straightforward to update or
add processing modules to handle the additional information from
the new detectors.

We will discuss the general structure of the Offline Framework,
explaining the design decisions that provided its flexibility and
point out the few of the features of the original design that
required deeper changes, which could have been avoided in
hindsight. Given the disruptive nature of the AugerPrime upgrade,
the developers decided that the update for AugerPrime was the moment
to change also the language standard for the implementation and move
to the latest version of C++, to break strict backward compatibility
eliminating deprecated interfaces, and to modernize the development
infrastructure. We will discuss the changes that were made to the
structure in general and the modules that were added to the
framework to handle the new detector components.