[tsimionescu]

I think the only way to end up with a successful re-write is to be extremely realistic with the whole organization about the scope.

For a program that took 10 years to develop, a rewrite will probably take around 10 years as well to reach feature parity. Obviously, if you stop development on your successful app for 10 years waiting for the rewrite, you will fail. If you think 10 years worth of work can now be finished in 2 years with the benefit of hindsight, you will fail.

But, if you start your new app by first attacking new markets that the old one couldn't, so that the new app is genuinely valuable in itself, and then you slowly consolidate it by adding features that the old app had, while still maintaining a crew developing the old app as needed to address the existing markets, you can justify the investment and then some. Of course, this should only be done if it has become clear that the old program is really not possible/worth it to extend and continue in the long run, which can happen for various reasons.

There will be times where you will find massive shortcuts you can take, since odds are things you had to develop from scratch 10 years ago exist pre-packaged today. You may also be able to re-use parts of the old app that were actually in good shape and take them whole. But you should never rely on these things making your work shorter - they are nice boons, but the correct estimate is still in the order of magnitude of the old app.