its impossible to say exactly what impact it would have without a proper assessment, but i can say that the efficiency of the boiler is one of the main factors in determining your rating...
if you are down around the E1 levels.. .a 95% boiler may jump you up a few levels...
if your around the d1/c3 level.. it might only jump you up one level...
but suffice to say that if your existing boiler is 70% efficient and your new only will be 95% efficient.. then you will be making 25% savings on your heating costs...