Here's one we wish we had thought of a few years ago. A professional tax package isn't useful unless it does some minimum number of forms, and the minimum is very large. So building a new Windows-based professional tax package is a multiyear investment. And there's just no way to release just part of it.
Well, if your company already has a DOS-based package that's comprehensive, how about putting a Windows GUI on top of it, and using that calculation expertise until you get the more powerful object-oriented incremental calculation engine working?
How about using individual forms calculations from the DOS program and replacing them incrementally with the new engine?
How about using the existing forms print capability of the DOS version, ...