[tweetiepooh]

It should be possible to slowly replace things if the systems are modular enough so that specific function is replaced in non-COBOL piece by piece.
My main concern would be on the data stores, programmes are easy to change but the data store is often less so. If you can mirror out the data store, ideally two way then the new platform can work with the new datastore with transactions mirrored to the old and visa-versa. The problem in some areas is latency and even millisecond delays can cost plenty but surely if everyone is equally affected????
It's not trivial even so and you know full well that things never work out as planned, especially as it gets bigger and you have multiple layers of project managers all messing things up.