.NET Case Study - Cannot decide on a pattern.

Hello all,

Following is the case story.

An organization has an application created using a proprietory
workflow design and flow package, COM objects for passing messages and
data to and fro and an ASP/VB UI/Business layer.

They need to upgrade the application. During the interviews the CTO
expresses interest in latest technologies. The decision to use .NET
has been made (strange but thats what it says in the case study). The
idea is to replace the business layer and the UI layer.

What kind of pattern would we be talking about here?

The obvious choice is "Bridge", but my question is, is there a
specific pattern (.NET architecture pattern maybe) which exactly
reflects upgrading from old version and interfacing with a legacy
application (The workflow engine in this example)

Anybody has any ideas/suggestions? let me know.

Cheers