To be super clear, when compiling with /clr, everything that you could do in
your code before you can still do. The /clr option only enables you to do
more.

If you have a function F compiled with /clr, the Visual C++ compiler
produces two functions for F. One has the unmanaged calling convention and
one has the CLR calling convention. One of these functions actually does
work (i.e. represents the source code written for F), and the other function
thunks (a.k.a. p/invokes) to the other function. This enables any function
outside of a ref class, value class, or interface class to be exported with
dllexport.

Inside of ref classes, value classes, and interface classes, the compiler
only generates the CLR calling convention function.

There are numerous ways to control how all these features work. Reading
through the documentation is going to be your best help right now. For the
short term, I'd encourage just recompiling all your code with /clr to see
that it still works.

Hope that helps!