There is definitely a lot to be said for the sentiment "If it ain't broken don't fix it". If your application is working and Visual C++ 6.0 works for you then why change it. There is a story (probably an Urban Legend) of a development team in IBM that only used obsolete compilers - that way they knew that the compiler would never change.

Having said that Visual C++ 6.0 is a very old compiler (it was released in 1998) and it is no longer supported bt Microsoft - so if you do run in to a problem you won't be able to get any help from us.

Also a huge number of improvements have happened since 1998 - the C++ compiler itself is light years better (especially for templates), MFC has had heaps of bug fixes (a lot of which won't have made it into any Visual C++ 6.0 SP), there is all the security work - both in generated code and in the libraries, there is support for 64-bit platforms, and up coming is support for Vista.

Upgrading a toolset can be a pain - especially when there is such a huge differences between toolsets as there is between Visual C++ 6.0 and Visual C++ 2005 - but on the whole your application should come out of the process in much better shape.