One way I've seen this done is to have two application running side by side. Leave the Legacy App as is and build your MM App under a whole new project. As you rework your new application disable the feature in the old application. It'll take some getting used to by your users but they'll manage.

You have a much greater challenge than your user looking for a menu option.

Of course, if you have a "Shrink Wrap" application then it may become a little more difficult to do.

Good Luck!