Obviously, the new features may not (probably will not) have analogs in the RI, but where there is comparable functionality, use it.

This is exactly what I meant.

(And remember the first rule of System Analysis: "The System you are designing must function at least as well as the System it is going to replace.")

And this is exactly what I had in mind... in fact the whole question arises out of the need for the new application to work exactly as the old faithful one when no new feature is used.