A good one to prompt a rewrite is when the implementation technology is unsupported anymore.

I recently had the experience that Windows 7 (64 bit) explicitly said that it disliked the Visual Basic 6 installation cd when inserted. We used that as a strong hint that it was time to port our VB6 applications.

A good one to prompt a rewrite is when the implementation technology is not supported anymore.

I recently had the experience that Windows 7 (64 bit) explicitly said that it disliked the Visual Basic 6 installation cd when inserted. We used that as a strong hint that it was time to port our VB6 applications (to Java as that is what we do).