Client / server experiences

I recently had a discussion with a consultant who was approached by an organization interested in migrating applications developed in Cobol on HP-3000/MPE to Client Server. These are back office operations of a securities/banking company. The current application development has already taken a time of over 2000 man-yrs(?). Though the management wants this transformation of the application to C/Server, they are not sure of the ultimate gains i.e. whether the overall costs of up-front investment, re-engineering, attitude change etc will be justified from the kind of returns available from this application. I would like to get some comments on the approach to be taken and also the examples of similar organisations who have taken such decisions and how they created their objectives/ parameters for success; specially in non-mission critical applications. Any suggestions / comments would be invaluable. Thanks, R.Raghu Ram