Hi,
Even when (as you were told) your DOS applications will run in any ulterior version with no code change, you will have to consider that the only sense to change is to use the new and more powerful capabilities of the new versions.
What I mean is that the smallest change, that is to say moving to FPW, will implicate apply this capabilities. I remember when I did it, it significated a "quantum leap", basically because of the advantages given by the developing tools and the fantastic integration to the windows enviroment.
The movement to VFP implies the same concept, but the leap is even larger. I you have applications running in client systems, you will have to evaluate and spend lots of tests time before.
Anyway, I think is unavoidable. Sooner or later, you will fave to move!
Good luck!
David.