>If your change is to occur across 5 years, and if you have lots of intensive VFP algorithms, you may wish to stage your transfer, getting the UI transferred quickly while retaining detailed VFP stuff accessed via mtdll or even via residual VFP fat client apps. Not as fashionable, but it makes business sense.

This is exactly what we've done. With our pure VFP Windows apps, we rewrote the GUI in ASP.NET. The biz tier was recompiled into a COM component and changed to accept and output XML. A very nice transition into the .NET world. We've slowly been changing the biz tier, object by object into .NET as we have time.