<p>What are all the problem that you foresee in doing that.</p>

How big of a code base are you talking about?
Porting a little program (that is mostly non-templated C++ code) should be fairly trivial.
However I once had to convert 100.000 lines of template-using code from VC6 to VC2005, and it was a nightmare week(5 days of work), the main problem was that I had to fix by hand about 30% of the problems (70% were fairly trivial and could fix them with search & replaces). But more of an issue was the fact that the old code had no test cases and no test framework, so even after I got the application to compile and not segfault, and look ok(?), I had no assurance everything it was actually working as it was supposed to be.
So actually my advice is to consider the size of the code, and the availability of tests, and also consider if the code really needs to be ported (in my case that was a Yes, but its not always the case, especially if the software will be faded out soon)