How would you build on legacy system?

[Brett]

In 'Design Patterns Explained' 2nd Ed. by Alen Shalloway and James R. Trott,
on P. 55, he discusses a CAD system. It has a front end and back end non
object oriented system. The back end system is being upgraded. He creates
the new system using object oriented code. The old system is still
accessible to the company wanting the upgrade via the front end, as they
have much invested in the old system and can't simply do away with it. The
author puts a type of bridge in between the front end and two (old and
object oriented) back end systems. In this bridge is something he calls a
geometor extractor, which gets shapes from the back end systems and passes
them to the front end. He uses polymorphism at the geometry level but not
feature level. That's because the system needs to know which specific
feature it is dealing with.

Can some one elaborate on this system in general? If a system already
exist, how can you feasible design a new backend that works with the old
front end while also working with the old back end?

Thanks,
Brett