MS has never guarenteed that lib files from one version of thier
compiler would work with other versions.  However they generally do work
without a problem.

The question that comes up is:  Is this a regular Library or is it using
MFC?  Is it C code or C++ code?  Generally speaking if it just a generic
library it should be OK.  But if it uses MFC or some feature of C++
which may have changed between 4.2 and 5.0 then your users will probably
have to upgrade